[microbet]

He may well have you beat, but, with the 5 on the turn your check looks like you made the straight as much as anything else.

If he has the straight he wouldn't want to chase you off. If he has trips, well, either you have the straight or you don't. If you don't, you probably weren't on a straight draw and he doesn't have to protect. If you do, you will win. Either way, why should he bet big on the turn?

Maybe he had the best hand and just decided it was time to take it down, but I think it is more likely he was making a move or thought he had the best hand with one pair.