But a truck or a van doesn't, and the wind can push on those hard enough to tip then over or push them off the road.
Sounds like a job for Myth Busters. Although the photograph in question with the trail in the dirt running up to the rock, and no vehicle tracks, footprints, etc. would also seem to support the theory. Or maybe leave a cheap webcam triggered by motion on a tripod (VERY WELL ANCHORED!) by the rock and wait for the next windstorm might confirm it.