Various reasons, such as hormonal imbalance, pesky parasites, skin problems and hormonal imbalance cause dogs to chew their tails. Calm and trained dogs who have never showed interest in their tails before may have skin-related problems, like allergies, crusts or even bleeding in that particular area if they start to chew. When the dog is biting near the base of its tail, it may signify a problem his anal glands.
Pesky parasites can also congregate in the dog's tail and cause itchy fits. The parasite can be fleas or bugs, feasting on the dog's blood, or tapeworms, which can cause anal itching. Most dogs try to relieve the itch by biting or nibbling on the base of the tail. Dogs with hip injury or that have had spinal or skeletal injury may also manifest their pain through chewing on their tails. If chronic pain is suspected, an owner should consult a veterinarian immediately for diagnostic and treatment procedures.
Sometimes, dogs chew their tails because they are bored. Running in circles and biting the tail may be a dog's way of entertaining itself while his owner is busy or out of the house. If the veterinarian cannot find anything wrong with the dog's health, the action may also be due to anxiety, frustration or other emotional problems.