Well since the rear wheel doesn't steer, it follows the front wheel on a different radius. Most of a rider's weight is on the rear wheel as well, making it more prone to this kinda stuff. I've missed things with front wheel only to run right over with rear wheel, inclding a 3-4 inch chunk of concrete from a matching hole in the road nearby, nearly crashed, foot slipped & toe dragged on ground jamming the pedal into my ankle nice and hard. Isn't physics fun?!?!

- Andy