I asked my 10 year old daughter why my rug (in this case on top of carpet and just inside of the front door) always moves in the same direction (towards the front door) even though I obviously walk over it in each direction the same number of times. She said... "It's because of the way the carpet fibers under the rug all point the same way. When you step on the rug, the carpet fibers underneath get squashed down and when they spring back up again they push the rug in the direction they are pointing." She then illustrated this by scraping her finger across the carpet to show that if she scraped in one direction (opposite to the direction that the rug moves) her finger left a trail in the carpet but left no trail if she scraped it in the other direction.