Another vote here for the pinched tube theory. Getting a bit of the tube caught under the bead is actually pretty common, and the resulting blowout can happen any time from a few minutes to weeks after. In fact, I remember one guy in my club whose new bike was getting blowouts on every ride. The shop would replace the tube under warranty only to blow the next ride. Finally a couple of us grabbed his bike before the ride and reseated both tires. No more blowouts. Moral: even shops can screw up and pinch the tube.