The spawn waves are on a timer so they won't overlap with the next group, this guy's timer ran out. I have been charging in on Dregs and Vandals in the Divide, in broad daylight, and had them dissolve into thin air in front of me. Then the next shipload of them arrived. I believe this is because, if the waves just accumulated and nobody were around to kill them, there would be like 9000 Vandals standing around smoking cigarettes and waiting for the first Guardian of the day to show up so they could kick his/her ass.