yeah, it does happen at random. I think its the old glitch from W@W -- it starts to glitch up when you camp in the same spot every round (by camp, i mean stay in the same area). Basically the zombies just pile up for some reason.... nothing too particular. Only thing is, in W@W they all stopped coming in at about round 30. I've been to round 37, and they still keep coming, so i think that infinity or treyarch (dunno?) have done a half-arse job at patching this.