I checked the turn because I thought he might have called my flop bet with something other than a draw, maybe a weak top pair. I'd seen him do that & call people all the way down, only to beat their middle pair/good kicker, or lose to their top pair/good kicker. I figured if he had what I thought, no bet would chase him out except a really big one, and I don't like semi-bluffing like that unless I feel there's a 99% chance he'll fold. So I checked it through to take a free card, and hit my draw - just in a way that made me nervous. Particularly when he sprung the big bet on me.