If you look at the pattern of a front tire compared to the back, the tread pattern flows in the opposite direction. They started to do this several years ago for better traction up front. In short part of the reason is rear tires require more traction on acceleration and front tires need more traction on braking... So jump to today and crazy riders (sarcasm) put rears on front so running it backwards replicates modern pattern of front tires...