I"m guessing that the cones came new too tight (very common). If the bike is a UO-8, bearing quality is not especially good so there may well have been tighter places within the rotation of the axle. The balls may not have come from the same batch and one was slightly larger. Now add rider weight. That large ball gets regularly "squashed". After enough of these, it breaks. (It's also done some damage to the cone.)

I'd buy some new bearings, put them in fresh grease, tighten just until play is gone with the wheel on the bike and the QR tight and ride it. (Note that this means the axle will have a touch of play out of the bike.)

Ben