my 12K quarenteed

I payed a 12K guarenteed a couple of days ago, and had come about 2 hours into the tournament. I had a wealthy stack of about 20K.  
I have AK and raise preflop with one caller calling with A5.
The flop comes A 10 2, and i raise 60% of pot hoping for a call. Not only does he call me, but goes all inn. I call instantly.

River comess out 5 and im out of tourney. His reasoning for going all inn on me, was because he thought i was bluffing and wanted to take down the pot on the flop!