Dr. Horwitz, a Veterinarian says:
There are two common reasons that a dog might "scoot" their bottom along the floor. In most cases the problem is due to blocked, infected or irritated anal sacs. The anal sacs are two glands on either side of the rectal opening that normally discharge when the dog defecates. These glands can become impacted and/or infected causing pressure and discomfort. Scooting behavior may often empty them. However, often they need to be manually emptied by the veterinarian. The other reason a dog may scoot along the floor is itching around the anal opening either caused by allergies or parasites. A veterinary examination should help determine which of these is causing scooting in your dog.