Let’s imagine NPC is running from A to B. The B is final destination until the NPC gets aggro. If I knockdown such NPC (with Tremor) and once the KD effect wears off, the NPC turns back and returns back to original spawn location.
It seems that the NPC lost waypoint (for path-finding) and needs to return back for getting back to track. Of course, I have no idea how is working your path-finding but it really looks like that knockdown effect breaks navigation. This is very visible in any location where NPC needs to reach specific point.
For example: Penitent camp where generic NPC’s are attacking the camp every half minute or so. You just need to knockdown that NPC before it reaches the destination in a camp.
It’s very easy to reproduce.