Why Do Dogs Spin On Their Butt?

Although I have never seen a dog  spin on its butt, I have seen them dragging their butt across a rug. They are actually rubbing their anus to relieving the itching caused by some species of worm.
Collect a stool sample and have your vet check for parasites.

Their anal gland may be full and need emptying. Ask a vet but you can look on Internet to see how to do it.