I was going to type exactly what Omax just said. I'm guessing that he but you on a strong ace or a flush draw. Since it was a bounty he is going to be more likely to call with bottom pair if he thinks he has a good read on you. As it is, he made a bad play and got lucky. As the board played out there, if you did have AK and only the first 2 hit, he still wins the pot. So I'm not sure it is a donkey play, but more of a bad read and hoping to be ahead of a draw or strong ace. Whatever it is, it sucked, you will have stuff like that happen again in your poker life, acknowledge and move on. Hopefully you'll play against that guy again and he will continue to make poor reads and the percentages will again be in your favor.