A. Your dog possibly scoots because his anal glands are bothering him. (as stated above) When you see him scooting his tush on your favorite carpet, he may be looking for relief.
Both dogs and cats have two small pouches located on either side of the anus called anal glands obviously. These glands are emptied during your pet's bowel movement, in an effort to mark territory, or as a reaction if the dog is frightened suddenly.
Have you ever seen a dog scooting on the ground at the dog run? This may be the same thing -- a dog trying to "clean" himself, if you know what I mean. The situation usually resolves itself quickly but if it is a constant issue... go to a vet. Also speak with your veterinarian about a possible diet change can help this happen less often.
If your dog is scooting around more than a vacuum cleaner, take it to the vet right away for a full evaluation. The dog could have an anal sac disease -- these are more prevalent in smaller breeds. Lastly, though not as common, your dog could have a tumor known as adenocarcinoma.
Sometimes this behavior has nothing to do with health at all. Your dog may have a foreign object near its tail, like a twig, for example. If this is the case, don't try to dislodge it -- you may do more harm than good.
Sorry not trying to scare anyone.
If you're ever unsure of what to do, call your vet. Remember, rug burn isn't pleasant either!