Most Protestant scholars believe in justification by faith alone (obviously). And there is also a tendency to extend faith alone further, i.e. into sanctification too. Beginning with this doctrinal bias, they start with the presupposition that Romans and Galatians are the unquestionably authentic epistles because Romans and Galatians are the most useful to their own theological perspective (i.e. faith alone). Then they notice how totally foreign that perspective is to the pastorals, not just Titus but even moreso the two epistles to Timothy.
Secondarily, it is well known that Marcion of Sinope (in Pontus) did not include the pastorals in his mangled edition of the Pauline Corpus. According to Tertullian, he included all the Pauline epistles other than 1st and 2nd Timothy, Titus, and Hebrews. Each epistle, other than Philemon, was shorter in his edition than in the Catholic edition. He is accused by Tertullian, Ireneaus, Epiphanius, and several others, of having purposefully made deletions from the Pauline epistles and the Gospel of Luke for doctrinal reasons, particularly his distaste for Old Testament citations. See Tertullian's Five Books Against Marcion (both Latin and English texts can be found at this link), particularly Book V in which he deals in depth with Marcion's edition of the Pauline Corpus.
From the end of Chapter 1 in Tertullian's fifth book Against Marcion (Holmes' 1870 translation):
We have laid down this as our first principle, because we wish at once to profess that we shall pursue the same method here in the apostle's case as we adopted before in Christ's case, to prove that he proclaimed no new god; that is, we shall draw our evidence from the epistles of St. Paul himself. Now, the garbled form in which we have found the heretic's Gospel will have already prepared us to expect to find the epistles also mutilated by him with like perverseness--and that even as respects their number.
And from Chapter 21:
To this epistle alone (i.e. Philemon) did its brevity avail to protect it against the falsifying hands of Marcion. I wonder, however, when he received (into his Apostolicon) this letter which was written but to one man, that he rejected the two epistles to Timothy and the one to Titus, which all treat of ecclesiastical discipline. His aim, was, I suppose, to carry out his interpolating process even to the number of (St. Paul's) epistles.
(The word "interpolare" translated here as "interpolating" seems to be being used here in the sense of "meddling" rather than "adding," since Tertullian is telling us he left these out.)
Of course, using Marcion's edition against the Pauline authorship of the pastorals is dubious, since Marcion is known to have tampered with even the epistles that he retained.
Thirdly, scholars read the pastorals as being directed against a Gnostic theology that many of them tend to doubt existed in Paul's time.