Just like the situation with the golden-winged roc, similar occurrences were happening throughout the demon realm.
Any body that was already filled with the "power of death" was in the process of giving birth to that eerie corpse power.
Even if they still had some life force left, it was no longer able to activate other parts of their bodies, which essentially meant they were "dead."
And these "dead" bodies, under the corrupted authority of the corpse path, began to revive.