The believers to whom Paul preached the Gospel are to a certain degree - and a very significant degree, for Paul even calls himself their "father" since he gave them new birth in Christ through this preaching (1 Cor. 4:15) - his husbandry, his cultivation. Now, if he sees that his cultivation bears no fruits and disappoints him, this disappointment he calls "God humiliates me through your fruitlessness". As a coach of a football team is humiliated if his team loses, so Paul feels himself humiliated if his cultivation, his children in Christ, bear no fruits.
But why God? Does God humiliate anybody? Whenever we use any term with reference to God who is infinitely good, then all terms change semantics. Yes, in a way God humiliates, for He humiliates the proud (Psalm 18:27), He inflicts a punishment of heavy illness even upon Paul (2 Cor. 12:7), but all this He does out of love, in order to help a haughty person to repent and become humble, and in the second instance, to prevent Paul from becoming haughty by the excesses of divine revelation in him.
So, here also, Paul glories himself in his cultivation, in the people Christianized by him, but he fears that it is possible that his expectations may deceive him and his glorying himself will belie itself, and by this God will humble, humiliate him, so that he may work even harder for edification of the church, not glory himself too hastily in them, but be more realistic. Thus, even this humiliation will be for benefit of Paul and the church of Corinth, but Paul proleptically writes to them so as to avoid this humiliation and have them rectified.