Just another side effect of Magic. No traveler who has encountered one has experienced a hostile encounter without having provoked it first. Since they can remain in the same spot for long periods, most of the time they are not even noticed.
With enough Magic Essence around, a pile of rocks, ice, dirt or other solid elements can gain conscience for some reason. They can be created with magic to follow simple commands, and since they can rebuild themselves around their faces, using them as protection is a wise choice.
