There are a few possibilities: an old trauma that now is more evident because the fur is falling out and not being replaced (usually a temporary condition, however I've seen some permanent baldness in severe injuries), an ongoing self-inflicted condition such as excessive licking brought on by itching or a psychological problem (one cannot observe an animal twenty-four
hours a day, and even though you may not actually see this behaviour does not mean it is not happening).
Itching can be a self-reinforced problem from an old insult, or any number of other conditions that are ongoing. As far as treatment is concerned, I would suggest determining the type of condition before any medication is used.
If this were an old injury, then no other treatment is usually indicated. If this has a psychological basis, then it will only be revealed when physical causes have been ruled-out. I wish that I could be more specific for you, but I believe that a veterinary visit is in order to answer your question satisfactorily.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124