ive seen this b4. could have just been loose and the pressure after a while one twisted its way out
since the wheel was loose, but maybe not loose enought o completely come off, it just wobbled along lightly after a while those lugs will lose strenth til it got to that point.
when my wheel came off my old car my studs (on jap cars the car has studs/lugnuts rather than bolts) were broken just do the rotational force with the wheel just wobbling on the hub.
id still call it loose lugs that overtime gave out