Thomas Aquinas in his Summa Theologiae said "God does reprobate some.". But the question is how does he do it? God is not the author of sin, nor does He make anyone to sin. So how does He do it?
IMHO, it is simple. He lessens the restraint (boundaries) on their sin and lets them fall further into the corrupting nature of sin on the own.
A good example of this is Pharaoh's opposition to letting the Hebrews go to worship in the wilderness. After one of the ten plagues he would repent (a bit) and then turn around and oppose God yet once again. Romans says that God hardened Pharaoh's heart.
Again, how did God harden Pharaoh's heart without being the cause of his sin? My belief is that God moved some of the boundaries against evil and Pharaoh simply fell into it. Thus Pharaoh was responsible for his own disobedience and yet God's will was done.
How marvelous is God, far more subtle than man and whose ways are past finding out. But in some, we have a few clues.
I agree. The question isn't really why does man fall into sin. As you've stated, God withhold His hand and we fall into our corruptible state. The real mystery is why does God saves any of us. With predestination we tend to focus on those who are destined for hell when we should be focus on why God has destined some of us for heaven.
1Ch 17:17 And this was a small thing in your eyes, O God. You have also spoken of your servant's house for a great while to come, and have shown me future generations, O LORD God!
1Ch 17:18 And what more can David say to you for honoring your servant? For you know your servant.
1Ch 17:19 For your servant's sake, O LORD, and according to your own heart, you have done all this greatness, in making known all these great things.