My player characters killed some fire giants near an evil altar and then left the area. When they returned, I told them the fire giants were zombies now.
My players were confused. They kept wanting to know who cast the spell.
I told them nobody cast the spell. If a creature dies in a horrible way, especially in close proximity to a source of evil, and if the corpse isn't properly buried or given last rites, then sometimes you just get undead.
Is there any rules precedent for this?