The walking dead are strange things, animated by hunger, need, rage and a touch of magic. In Ashenfall it is the hunger of Imaru that drives them, as her great maw draws in the souls of both the living and the dead, the afflicted try to cling to life with everything that they have.
They fail, but the drive remains. Fear, desperation, and simple need keeps the body moving, if not alive. This changes the body, that hunger and rot seeping into the very bones and changing them to be something other than they were.
When ground down, these bones become a powder containing hints of that animating magic. A necromantic resonance with a wide range of uses.