I've been watching the History Channel - How the Earth Formed - series. I recommend it to everyone... anyway, two weeks ago they did the sliding rocks. A NPS official keeps track of their movement with GPS. The theory is because the ground is so slick when it rains and it is located in a wind tunnel running between the two mountain ranges around it that if they have high winds after a rain they actually slide pretty easy - think those sliders you buy to put under furniture to make it slide like glass.