No kidding. Two weeks ago, a AA battery that I'd taken out of my digital camera a few days earlier (and placed in my car's little catch-all on the console) exploded while I was driving down the road. Who knew??
My ear (and the side of my head) hurt for two days from the noise.
I'll bet you had some other odds and ends in there with it, like coins or keys, which managed to jostle around and electrically short-circuit the two ends of the battery.