If you fall and skid to a stop on a carpeted floor, you can get a rug burn. Much of this discomfort comes from abrasion, but there can also be a real burn where the skin was too hot. When running, your body has energy.
When you fall, the friction between the carpet and your body transforms your initial energy into energy.
Because the surfaces in contact (your knees and the carpet) are heating up, it can result in a burn.