There are several possible reasons for hair loss in dogs. As you first suspected excessive rubbing/scratching can cause localized hair loss as can flea/mite allergies, mange and hormonal imbalances. The ailment that seems to fit best with your dogs symptoms is ringworm which is an easily treatable fungal infection. It is often detrimental to the rest of the coat, as well as causing patches to fall out. It is hard to diagnose however without seeing the animal and it is possible that the ailment is mange or one of another dozen possibilities. A veterinarian can do a examination/skin scrape to get a definitive diagnosis. It is important to treat both ringworm and mange very quickly as both are highly contagious.