Hello! My name is XXXXX XXXXX it will be my pleasure to help you with your dog today.
Dogs chew at their rumps on the ground for several different reasons:
First, they may actually have intestinal parasites. Dogs generally get 4 kinds of worms: hookworms, whipworms, tapeworms and roundworms, and you can't always see them with the naked eye. If a dog isn't actively shedding the adult worms, you need a microscope to determine if they even have them. A fecal sample brought to your regular vet where they can use a special solution and a microscope to look at the sample is the only way to know for sure if your dog has worms and/or what kind they might have.
Secondly, full anal glands. Dogs have two small pea-sized glands located just inside the rectum. In the past, they were used to scent mark so that other dogs would know where they were and what territory belonged to that particular dog. These days, those glands aren't necessary in tame dogs, but they have them anyway. The glands are filled with a foul smelling, oily substance and can sometimes become impacted or abscessed. Even dogs who have never had a problem with their anal glands in the past can get a problem with them, so having your vet check them to see if the glands are full is a good idea. If the glands are full, the vet will manually express them so that they're empty again. Once empty, the dog will generally stop bothering their bottoms.
Finally, they may have an irritation in the anal area such as dried feces or other things that can cause itches. If you can't see a problem with the naked eye, then the best thing to do is to have your vet take a peek. A quick fecal test and checking of the anal glands usually finds the problem.
I hope this helps.