The locals in deadwell insist that the reason the undead keep appearing in the crypts is that adventurers keeping going in and disturbing the rest of the dead.
The undead literally do not wander past a certain threshhold from the graveyard which may lend credence to the theory.
I personally suspect that the undead are the byproduct of a certain wizard's experiment- but the evidence for such is non-existent.
Grace