Why do cats drag their butt on the carpet?

This is a big sign that your pet's anal glands need to be expressed. The vet or groomer can do that.
Or they might have a tape worm in which case you need to get some medication at the vet.
Or sometimes, faecal matter might get stuck on fur especially in long haired dogs and cats- if this is the case just get it clipped at the groomers.