If this is the ONLY spot she is biting, it may be pain? I'd have the vet x-ray her back and tail to make sure that this area is OK. If you are near a physical therapist for animals, you could have her evaluated for low back pain.
If you are feeding a grain-free diet, and that does not resolve the biting (it may take 4-6 weeks) then I'd go with the above. Have you ever had her in a collar that makes it impossible for her to get to her back? I have a rubber inflatable one that is nicer for the dog, than the regular plastic 'cone'.