I don't know, but I do know that the way I see it is this:

When you "call" a bet, you are paying for the right to see if a hand is better than yours. He clearly thought he was good, and casually threw his cards in. Dealer politely points out that he was called, and has to showdown. He had a better ace, and won. He didn't muck in frustration like some do, but just threw his cards not realizing someone paid for the right to see if he was good.