I'm looking for a little insight into an odd thing that happened last weekend:

Saturday evening, we suddenly hear a loud popping noise from the room in which the bikes live. It turns out that the tube in the rear tire of my almost-new bike had suddenly burst. Upon examination, I can find nothing wrong with tire or rim. The tube is of course shredded.

I had the bike out for a ride of about 15 miles Saturday morning, but it had simply been sitting for at least 8 or 9 hours when the tube burst. Tires were aired up to recommended pressure (120 psi) before the morning ride. Bike had not been sitting in hot sun or under any other sort of extreme environmental conditions. Bike really is almost new; has probably about 150 total miles on it so far.

Took the tube, tire, and rim into LBS to see if they might see something I didn't. They also found nothing wrong with tire or rim, and had no idea why the tube might have burst.

What happened here? Was it just a bad tube? Is there another possible explanation? I've only been riding seriously for a couple of years, so I know there's a lot I haven't seen.