It is usually either of two things:
- player 1 has a great made hand, and hopes to get his opponent to raise him.. since this hasn't happened by the river, he still wants to cash in and bets big.
- player 1 has a decent hand like TP, but is scared of something, like a draw. So he bets small b/c he is too afraid to bet big b/c of the possible draw or something. But by the river, his scare card hasn't shown up, so he feels safe that his hand is best and bets big. (could also be MP when he is scared of TP.. by the river his opp hasn't raised him so he thinks he is good)