Cat wipes its bum on floor

There are two common possibilities here:

• Your cat is just trying to wipe its butt. It happens, especially for long haired cats (but our shorthairs have done it once or twice) when they didn't manage to "pinch it off" the way they usually do. Maybe the poop got stuck in his fur, maybe he's constipated and couldn't quite lay his poop.
• There is something wrong. A common cause is worms causing an itch.

However, since you mention this happened straight after using the litter box, the former seems more likely.

Keep an eye on your cat and see if it keeps happening (or if any other strange behavior occurs). If it doesn't, it was just a bout of constipation or diarrhaea. If it does, consult a vet for a proper examination.

It is unusual for a cat to rub his butt on the floor, something is clearly wrong.

It sounds like your cat might have an infection or an injury. Cats have some glands in the anal area that might get infected. Your cat might be in pain, so you will have to take him to the vet.

Another cause for your cat to rub his butt against the floor might be intestinal parasites, as this can itch a lot; get your vet to deworm your cat at the same time as you take him to the vet.

At 9 years old your cat is in the beginning of his senior age, so you should start to take him to the vet every year for a checkup to discover any age related problems before they get more serious.