Check her carefully for fleas. Not sure if feet just seem to be itchier or if fleas really do bite dogs' feet more, but it's a common symptom. Also, carefully and gently inspect the foot she's chewing on. There could be something stuck in between her footpads that she's trying to remove.
Usually when they chew their feet it's because they're itchy. If it seems like she's doing it during a certain time of the year, it's likely allergies, and it isn't a big deal to treat if you take her to the vet.
The obvious would be because it is bothering her. It could something as simple as a splinter to arthritis of the joints in her foot or bone disease .. Or maybe even she needs her nails clipped. No one can assess the problem .. She needs to be examined by someone qualified to do so.