If, as suggested, the dog doesn't have any physical problems with his feet, you'll find that dogs, as well as other domesticated animals, often exhibit this kind of repetitive syndrome. It seems to be just a way of dealing with stress. We had a German Short Hair retriever that did this with the addition of occasionally chewing his toe nails. Repeated visits to the the Vet indicated no health problems with his feet (or anything else). An older Vet said his experience was, as I described above. We moved a recently acquired cat out to the barn and, voila, the dog stopped the chewing... check him for health problems though, as wisely suggested...