hi, my bitch does this as well but the top of her feet and nails. it has gotten to the point where she has to now wear a “cone” everytime she bites her own paws, simply for her own safety as now she has a bad wound trying to heal. I did some research on this and it seems they do it as a start to remove grit, dirt or obstacles within their paws and it can become an obsession. I have found having plenty of things to chew hanging around can help a bit.