Move in, pull a group of hostile NPCs (generally dungeon trash), and while some of them move in or remain in melee range, others remain or move away and engage at range (often in opposite directions). Yet others start out in melee range, disengage, move out to range, and throw something at you. This can pull yet more hostile NPCs, and the whole process repeats.
I have not seen this mechanic in other games. It's a really simple idea, but it makes "trash" fights at least incrementally more interesting. In difficult content, where enough "trash" can be dangerous, you actually have to think about your pulls :-).
Weird side-effect: when things move out to range, disappear into walls, and can't be reached/damaged any more. In effect, the NPC leverages an "exploit", and "LOS's" the player.