Katina Tamihana, of Manawatu College, asks :-

Why do dogs drag their bottom over the ground?

Rachael Stratton, an animal behaviourist Massey University's Institute of Veterinary, Animal, and Biomedical Sciences, responded.

This is called scooting. Usually dogs do this because the area is irritated for example from intestinal parasites (worms), full or inflamed anal glands, skin or anal irritation from diarrhoea/dags and rubbing it on the ground (or carpet) scratches the itch that they can’t otherwise reach.

Perhaps some dogs do it because we laugh at them and give them attention like some do with tail chasing? If scooting is persistent, you should have your vet check your dog's anal area and anal glands (which normally empty when they pass a bowel motion but can become blocked and infected).