Recluse spider bug thread reminded me of this. Last week my pc and three others took the tithe herd quest at the spire. When we entered we were immediately confronted with a small horde of enemies- magic eaters, deep hounds and deep hound boss. We fought through them and killed the boss, which completed the quest. 20 seconds later the rothe appeared, which suggested that the quest was going to begin anew. We didn't stick around to see if another boss npc would spawn; we left and closed the quest instead.
I wasn't sure what to make of it at the time but now I think the problem stemmed from the previous group who had taken the quest. Since the enemies were camped right on transition, I think what happened is that the previous group abandoned the quest, but that didn't seem to get rid of the remaining enemies.