Minor annoyance: Whenever I pay the fine to a guard (for the first time) to avoid going to jail (and remove bounty), I spawn outside the nearest castle jail. Along with me, appear a bunch of mod-added characters depending on if I have these mods installed - The ship captain from Cybiades, one of the added NPCs from Heroes of Cyrodiil (an Argonian), a ghostly king from The Sundered Keep, and on this install with WAC, an armored Xivilai who is an ally. Chaos ensues as they have a fight till death. Guards also join the fray. Good fun only the first time, minor annoyance in later games.

Not sure what causes this, but it has happened to me since time immemorial. Is is a setting for NPCs to appear that gets triggered on a jail cell or fast travel?

Could be it would happen on fast travel as well, but I haven't done that in eons, so can't tell. Does this happen to you? Is there a fix?

I have this with a character from Path of the Skull Bearer too. It must be similar to the old Fighters Guild problem where the 3 guild members from the Desolate Mine showed up after you get sent to jail, no? That was fixed in the UOP.