This is either a bug, or respawns happening too quickly.
This Risen in the graveyard continue to spawn at such a rate, we could not kill them all. I thought we had it down to 0 a couple times, but another would pop up, and then many more before we killed them all. I think I was on that step with a few others for around 15-20 min, until we eventually failed the event.
The only outcome of this event seems to be failure. If this is not the case, then perhaps the spawn rate needs to be toned down a little.