Dog Biting Tail Allergy. She could also be allergic to her food or treats, or could have been exposed to something in her environment that she's sensitive to, like a cleaner, laundry detergent, or carpet spray. Then in terms of food, it could be wheat, carbs, vegetables and even some forms of protein.
The second most common reason for dog tail biting is allergies. Since dogs cannot talk, they cannot make their needs or desires plain to us in any simple or intelligible way. Obsessive or compulsive canine behaviors, like chasing and gnawing at the tail, always have a root cause.