According to William of Newburgh, it was a fairly common occurrence for undead corpses to leave their tombs and terrorize villages in 12th century Europe. Why are there many supposed eyewitness accounts regarding the undead during this time?

by soggypoophair

Edited to add a quote from William found on wikipedia--

It would not be easy to believe that the corpses of the dead should sally (I know not by what agency) from their graves, and should wander about to the terror or destruction of the living, and again return to the tomb, which of its own accord spontaneously opened to receive them, did not frequent examples, occurring in our own times, suffice to establish this fact, to the truth of which there is abundant testimony.

BigfootSF68

Using what we know now to be hoaxes, are there any contemporaneous documents that refute the author's statement? Or should we view all of his work with the same eye that one provides conspiracy theorist books today?