The respawn timer on Chtonic Knights, Inquisitors, Crawler, Nightmares, and basically all DF endgame trash mobs below Essence Shredders is so quick that a small group has a hard time clearing the area before getting swamped with respawn. Especially Theurg pets, which the Theurg is missing an "all release" spell for, tend to draw aggro from the respawning mobs.
They also aggro from insane distances and draw adds from mob groups behind walls. Especially killing Dommel is basically impossible to manage without fighting respawning adds at least twice when with 5 players.
Suggestion: Modify respawn timer to reasonable numbers and decrease aggro range.