My brake pad blew right out of its housing today! Wow! And those were my good brakes. (I am fine)
It took me a while to realize exactly what happened here, (partly because I couldn't examine the pad, which flew off 30 ft. away, lost into a busy intersection), but I might have figured it out.
Based on how hard it broke, the way both the little metal flap and big metal caliper bent outward substantially, also based on just how worn-out the other pad was, I think that probably the failed pad wore down all the way to expose a bit of the bolt thing buried inside the rubber—the hidden other end of the piece you use to attach the pad to the caliper. So, a steel edge in the brake snagged at high speed on the softer aluminum alloy rim of my wheel. This began the event of the whole assembly bending outward, but wouldn't have been enough to grab and rip the pad out; more crucially, probably, it bent and tilted the pad in its place just enough to make it no longer parallel with the wheel rim, dramatically so, in fact, until the pad snagged again (all in an instant, of course) by the butt end of the pad and its housing being pulled forward. Basically the force of the wheel tried to spin the whole pad around, sort of turning the brake pad into a non-circular gear rotated against the driving wheel, a ridiculous movement that would certainly bend the caliper as much as it did. Obviously, the brake does not want to go that way, and the path of least resistance for it to escape this pressure was for the pad to bust out the little metal flap and fly away free into the sky.
The moral of the story is: keep your brakes in good shape, use both your front and rear brakes, don't be a fool like me.