The problem:
EDIT: I guess you could argue that's "the symptom". The problem is that there is a ball (half metal, half plastic) that fits inside a housing and the tolerances must be perfect for the housing to prevent the leg from falling off. It seems like sometimes the plastic ball is just a little bit too big, or a little bit of glue gets on the plastic part of the ball, and so the housing doesn't latch down strong enough to hold the ball together and the metal part just comes right out. I would think that an imperfection in the cast of the plastic half the ball would also cause the issue as that would cause the housing to separate a bit also.