The most common reason for a dog to scoot its behind would be what is called anal glands.
The anal glands have an internal sac on either side of the anus (approximately at 8 and 4 o'clock) that fills up with a smelly, liquidy substance. When you see your furry friend dragging themselves across the carpet, this usually means that they need their glands expressed. Anal gland expression, which can be done at home or at the vet, consists of placing your index finger inside of the dog's anus and placing your thumb on the left or right side of the anus. You will then bring your fingers together and make a rolling motion toward the anus with the index finger. Then you will repeat on the other side. Gloves and lubricant are needed, and paper towels for wiping the area down afterward are also useful.