who calls this to try for trips

[Tim H]

25 NL party
UTG is so tight his azzcheeks get raw when he has to walk anywhere

anyhow:
I am BB with 99
he is UTG with AA
he brings it in for $6 announcing to the world what he has

both stacks are around $60, I call
get the 9 on the flop to get paid.

Anyhow, he says I was wrong calling that when I knew I was a dog. I said I called because I knew what he had and $6 was only ~10% of my stack and its an easy fold once the flop came.

Anyhow, who calls pf? anyone?