Wierd stuff just happens sometimes. If the tube was old, I'd just chalk it up to an old tube, and go spend $5 or whatever on a new one and go about my business. If you have recurring problems, then it's something to get worried about.
A buddy blew out his rear tube once and we couldn't find anything wrong (no puncture, no bumps, wheel was fine, etc. The only thing we figured was that it was in the high 90's and we were riding across some pretty new blacktop that was probably significantly hotter than the air temp. We figured maybe his tire heated up enough that it increased the air pressure in his tube enough to make if fail. Can't confirm that this was the cause, but it sounded good at the time.