[shant]

I think becuase he has middle pair + backdoor straight + backdoor flush + the 2-pair outs are clean. So, he's giving himself 8 outs against a player he feels is loose with his preflop standards. He may have the best hand, and he has enough outs to see the turn, and the LAGgy guy just gave him the oppurtunity to try and thin the field. Since he's going to see the turn, raising is probably better than calling. Also, the pot is big and worth fighting for.

I think it's all those factors that make this standard this time, but not everytime you have middle pair.