Commerce 10-20, good laydown or too paranoid?

[Jeff2600]

This weekend, there was an interesting hand at my table, $10-$20 at Commerce.

Folds to MP, who raises to $100 ($1900 at start of hand), folds around to the small blind, who raises to $400 ($4000 at start of hand). Both players look like average solid players.

Flop is K 9 4 with two hearts. The small blind bets out $500. MP raises to $1000 (has $500 left). The small blinds thinks for a minute and says something like "you have to have pocket kings", and folds flashing AA. MP flashes 99.

When I saw this hand, I couldn't believe the AA folded. After he bet $500 on the flop, he was getting 2800:1000, almost 3:1 to put his opponent all in. But, after thinking about this hand for a long time, maybe AA made a great laydown. Since MP called the $300 raise preflop, I would put him on KK - TT. With the king on the flop, I would think that he would fold to the $500 bet with QQ - TT. It was possible that he wouldn't lay down QQ - TT, but the fact that he raised $500 instead of going all in looked like he may have flopped trips. Why else would he make that small raise? Does anyone think MP could have had AK? Would he have called that raise preflop with AK? Was this a great laydown or was AA being a little too paranoid? I would like to know what you guys think of this hand.