I've had bearing break on me -- my guess is a defective bearing. I bought a (very) used Canadian Tire bike (Supercycle -- equivalent to Walmart brand, I would guess) at a police auction. One of the bearings in the front hub was broken. I don't know the history of the bike, but the wheel itself was in good shape, so I don't think the ball broke due to excessive forces being placed on the wheel and hub (maybe the forces of the 50lb frame on the hub
). I guess my point is that it seems these things can break without taking excessive abuse.
The broken ball looked like it was made of shell wrapped around a smaller bearing of about half the diameter. It is this shell that cracked away from the inside 'bearing'. Did yours look similar?