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.
That's what I figured...the oddest things happen to me. It blew the case of the battery right off, and left part of the insides all powdery, and part of them still attached to the end (as you can tell, I'm a real battery expert...LOL).
It sounded like a shotgun...and you can imagine how loud that would be inside of a car!