Some Context First
One can only guess. My take is as follows.
Let's look at the context of what St. Peter is writing in this chapter (and Epistle in general)
2 Peter 3 (DRB)
Behold this second epistle I write to you, my dearly beloved, in which I stir up by way of admonition your sincere mind: 2 That you may be mindful of those words which I told you before from the holy prophets, and of your apostles, of the precepts of the Lord and Saviour. 3 Knowing this first, that in the last days there shall come deceitful scoffers, walking after their own lusts, 4 Saying: Where is his promise or his coming? for since the time that the fathers slept, all things continue as they were from the beginning of the creation. 5 For this they are wilfully ignorant of, that the heavens were before, and the earth out of water, and through water, consisting by the word of God. 6 Whereby the world that then was, being overflowed with water, perished. 7 But the heavens and the earth which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of the ungodly men. 8 But of this one thing be not ignorant, my beloved, that one day with the Lord is as a thousand years, and a thousand years as one day. 9 The Lord delayeth not his promise, as some imagine, but dealeth patiently for your sake, not willing that any should perish, but that all should return to [repentance]. 10 But the day of the Lord shall come as a thief, in which the heavens shall pass away with great violence, and the elements shall be melted with heat, and the earth and the works which are in it, shall be burnt up.
He explicitly says he is admonishing them not to forget that which was taught by Jesus (e.g. His coming as a thief in the night: Rev 16:15; Mt 24:43 etc.) and that one day is as a thousand years—the supposed 'long time' taken for God to return is nothing other than the shortness of our lifetime, not His slackness. Chief of all, that this perceived 'slackness' is time to be taken advantage of for repentance—His delay is "for your sake" at least in part. (See the next verse)
11 Seeing then that all these things are to be dissolved, what manner of people ought you to be in holy conversation and godliness? 12 Looking for and hasting unto the coming of the day of the Lord, by which the heavens being on fire shall be dissolved, and the elements shall melt with the burning heat? 13 But we look for new heavens and a new earth according to his promises, in which justice dwelleth. 14 Wherefore, dearly beloved, waiting for these things, be diligent that you may be found before him unspotted and blameless in peace. 15 And account the longsuffering of our Lord, salvation; as also our most dear brother Paul, according to the wisdom given him, hath written to you: 16 As also in all his epistles, speaking in them of these things; in which are certain things hard to be understood, which the unlearned and unstable wrest, as they do also the other scriptures, to their own destruction.
Where did Paul write such things as "account the longsuffering of our Lord, salvation; as also our most dear brother Paul, according to the wisdom given him, hath written to you?"
Well, something comes to mind right away:
Romans 2:4-11 (DRB)
Or despisest thou the riches of his goodness, and patience, and longsuffering? Knowest thou not, that the benignity of God leadeth thee to [repentance]? 5 But according to thy hardness and impenitent heart, thou treasurest up to thyself wrath, against the day of wrath, and revelation of the just judgment of God. 6 Who will render to every man according to his works. 7 To them indeed, who according to patience in good work, seek glory and honour and incorruption, eternal life: 8 But to them that are contentious, and who obey not the truth, but give credit to iniquity, wrath and indignation. 9 Tribulation and anguish upon every soul of man that worketh evil, of the Jew first, and also of the Greek. 10 But glory, and honour, and peace to every one that worketh good, to the Jew first, and also to the Greek. 11 For there is no respect of persons with God. ...
Cf. Mt 24:13. Luke 21:19. Rev 16:15.
When, then, did St. Paul write "of these things" which St. Peter mentions in general in this chapter (such things as walking so as to be found blameless in the day of the Lord, that God delays so as to give place for repentance, etc.)? In a great many places. Here is just one for each.
Walk to as to be found blameless by him.
Philippians 2:14-15 (DRB) And do ye all things without murmurings and hesitations; 15 That you may be blameless, and sincere children of God, without reproof, in the midst of a crooked and perverse generation; among whom you shine as lights in the world.
1 Thessalonians 5:2 (DRB)
For yourselves know perfectly, that the day of the Lord shall so come, as a thief in the night.
Romans 2:4 (DRB)
Or despisest thou the riches of his goodness, and patience, and longsuffering? Knowest thou not, that the benignity of God leadeth thee to [repentance]?
Which Things are "Hard to be Understood?" and Easily Twisted by the Unlearned?
It's easy to take St. Paul's words in many different ways, perhaps more so since, according to his own admission, he was "unskilled in speech." His simple mode of speaking readily engenders many interpretations—if context is not rigidly observed, respected and considered. By which I mean 'prooftext reading' is all to easy for the 'unlearned' to fall pray to. There are simply too many things, deep topics, on which St. Paul touches, to even begin to enumerate them all. Since St. Peter says that misconstruing the things he talks about specifically result in damnation "to their own destruction," this must pertain so a doctrine of justification/salvation, in my opinion (you won't go to Hell for assuming something wrongly about the date of the coming of Christ, for example). So...
One Example Looked At
I would consider the most woeful, notorious, but classic 'rending' of his words (at least in recent centuries) to be 'faith alone' justification (given the above passages, but also how verses supposed to teach it actually do not teach it positively) for one example, but there are so many things in the relatively large Pauline corpus which could be misconstrued not only by 'unlearned' Christians but by non-Christians of every variety.
Romans 4:5 (DRB)
But to him that worketh not, yet believeth in him that justifieth the ungodly, his faith is reputed to justice, according to the purpose of the grace of God.
'There you have it,' says the render of Paul's words (tongue in cheek), 'works don't enter into justification, only faith alone.' (Just as clear, if not just as much, as the single line quote in James: "You see then that a man is justified by works, and not by faith alone.")
This ignores even the immediate context which is contrasting a work based and grounded justification, over and against a faith based and grounded justification.
Romans 4:1-4 (DRB) What shall we say then that Abraham hath found, who is our father according to the flesh. 2 For if Abraham were justified by works, he hath whereof to glory, but not before God. 3 For what saith the scripture? Abraham believed God, and it was reputed to him unto justice. 4 Now to him that worketh, the reward is not reckoned according to grace, but according to debt.
What did St. Paul line these words up with? What specific view of works does he here condemn as regards justification? Obviously the view of works where they correspond directly to a just demand of God to grant entrance to heaven. Hence, "what is owed," and, "he has something of which to boast.' THIS IS A VIEW OF WORKS WHICH MAKES FAITH AND GRACE REDUNDANT! (Romans 11:6!) To 'work' in this context is to be a worker for a wage. Not to render obedience to God because "we ought" (Luke 17:10!).
So already, St. Paul doesn't even have in mind here the kind of Christian good works spoken of in the rest of Scripture, but a specific view of doing works which correspond directly to something you are owed by God—making grace redundant, as well as faith.
Something which helps drive this point through is reading what St. Paul points to: Abraham and his justification.
Since St. James speaks of the exact same justification of Abraham, even quoting the same Scripture, but says a little more about the event specifically, let's look at what the equally authoritative Apostle has to say about what St. Paul also refers to:
James 2:21-24 (DRB)
Was not Abraham our father justified by works, offering up Isaac his son upon the altar? 22 Seest thou, that faith did co-operate with his works; and by works faith was made perfect?" And the scripture was fulfilled, saying: Abraham believed God, and it was reputed to him to justice, and he was called the friend of God. 24 Do you see that by works a man is justified; and not by faith only?
Clearly his justification before God is spoken of (not 'vindication'):
James 2:14, 20, 23 (DRB) Shall faith be able to save him? ... But wilt thou know, O vain man, that faith without works is dead? 21 Was not Abraham our father justified by works, offering up Isaac his son upon the altar? 22 Seest thou, that faith did co-operate with his works; and by works faith was made perfect?
Obviously this is about the ability to save of faith alone, or "faith working through love" (Gal 5:6; 1 Cor 7:19). He isn't talking anywhere here of garnering respect or approval or vindication before men in order to make faith complete and not dead. For that can't happen: faith can't be complete only when other men see your works, if they are merely showing you have said faith! Which Genesis, to which he is directly referring, makes plain:
Genesis 18:12, 16 (DRB)
And he said to him: Lay not thy hand upon the boy, neither do thou any thing to him: now I know that thou fearest God, and hast not spared thy only begotten son for my sake. ... By my own self have I sworn, saith the Lord: because thou hast done this thing, and hast not spared thy only begotten son for my sake: 17 I will bless thee, and I will multiply thy seed as the stars of heaven, and as the sand that is by the sea shore: thy seed shall possess the gates of their enemies. 18 And in thy seed shall all the nations of the earth be blessed, because thou hast obeyed my voice.
Note that he says "Abraham believed God" was fulfilled when he did the work: "faith did co-operate with his works; and by works faith was made perfect.. And the scripture was fulfilled, saying: Abraham believed God.." He is defining the same faith as Paul as being completed by works, before which time it is then incomplete (yet both are viewed as a unity before God Himself, since He knows the works follow the faith, for which reason He can justify on the basis of the initial faith, and why the entire Bible teaches we can fail to do our duty and forfeit our friendship with God). It at this point in time became more than a promise to do good and go along with God's plan (branches but no figs), but a fruit. But it wasn't his doing this that made the promise itself. That was God. It was by faith that his works became meaningful, but still not deserving of or directly corresponding to the things rewarded by grace through faith.
As you can see, it takes a little time to put in the effort of reading the Scriptures quoted in some depth (context assumed to be known by St. Paul's readers by the mere introduction to a verse) than just 'ha! no works will enter into whether I'm saved!' which some are happy to haphazardly run away with and make the basis of a new religion entirely (making baptism optional or even reduced to symbolic terminology for 'faith,' the Eucharist not qualifying for salvific in any remote way, confessing your sins something of a helpful way to 'heal,' and not absolutely necessary, etc.)
I would say it is the single biggest misrepresentation of St. Paul to date, but again, St. Paul speaks in such a way (his palpable joy in the gospel, and his wanting to convey it in the most God-honoring way possible, including avoiding as much as possible the downsides) that it is easy to run off with a nice 'soundbite' out of its larger context—in a way not necessarily as true for other New Testament writers.
For example, you won't mistake what Jesus means when He says:
John 15:1-10 (DRB)
I am the true vine; and my Father is the husbandman. 2 Every branch in me, that beareth not fruit, he will take away: and every one that beareth fruit, he will purge it, that it may bring forth more fruit. 3 Now you are clean by reason of the word, which I have spoken to you. 4 Abide in me, and I in you. As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me. 5 I am the vine; you the branches: he that abideth in me, and I in him, the same beareth much fruit: for without me you can do nothing. 6 If any one abide not in me, he shall be cast forth as a branch, and shall wither, and they shall gather him up, and cast him into the fire, and he burneth. 7 If you abide in me, and my words abide in you, you shall ask whatever you will, and it shall be done unto you. 8 In this is my Father glorified; that you bring forth very much fruit, and become my disciples. 9 As the Father hath loved me, I also have loved you. Abide in my love. 10 If you keep my commandments, you shall abide in my love; as I also have kept my Father's commandments, and do abide in his love.
But every now St. Paul will of necessity, because of the specific point being made, sound more straight to the point, serious, and Jesus-like.
Romans 11:20-23 (DRB)
Well: because of unbelief they were broken off. But thou standest by faith: be not highminded, but fear. 21 For if God hath not spared the natural branches, fear lest perhaps he also spare not thee. 22 See then the goodness and the severity of God: towards them indeed that are fallen, the severity; but towards thee, the goodness of God, if thou abide in goodness, otherwise thou also shalt be cut off. 23 And they also, if they abide not still in unbelief, shall be grafted in: for God is able to graft them in again.
Point being that St. Paul touches upon many deep topics ("hard to be understood") sometimes with an almost childlike simplicity (sometimes not), and in simpler and less explicit language (and thus more susceptible to reinterpretation): to the degree that many who are eager already to believe some thing, will find in him a great advocate for their heresy.