[Pg9]

Because he has probably a straight with no redraws. He wants to get the money in while he is ahead. If he checks or bets small, and the turn flushes or pairs the board, he would had a hard time thinking he is still ahead, specially against two. He wants to go all-in right now (put you all in) and avoid tricky situations later.

Some people do the same with weak a holding like 2 pair and the NFD, hoping to push your "obvious" AAxx out of the pot.