I think there is at least hinted towards a possible "realistic" reason for the respawn in one part of the town (Aleroth), the one infested by undead ones.

It *could* be, however, that this single "source" also influences other parts of the town, too.

When you find a big kettle of crazy, it's best not to stir it.
--Dilbert cartoon

"Interplay.some zombiefied unlife thing going on there" - skavenhorde at RPGWatch