Clincher tires are required, I think, to be capable of sustaining twice their rated pressure before a blowout--and that isn't an unreasonable thing to be *able* to do.

Clincher rims, however, usually are another story. Lower end rims will blow out at around 100-130PSI, higher end clinchers at around 200 or so. That is all assuming perfect conditions. A defect in a rim could cause random blowouts at normal inflation pressures.

Probably, your brake rubbed a hole in the sidewall of the tire, or the tire wasn't properly seated with the tube and there was a pinch.