My guess is that villain had a mediocre pair or maybe even air in this spot. He bet only when you showed weakness (checking turn) and then when you slapped 'im with check-raise, he knew he had to fold.

Basically, by the turn, villain had control of the hand. He had raised massively on the flop, and you made what prob. appeared to be a crying call (which is the idea in trapping the sucker). On the turn, villain continued to try n represent a nice hand- bettin 9 after you checked. I think if you bet weak like $4-5 he would have been tempted to raise to like 12+ in order to buy the pot.