The dog may have a skin fungus or some sort of other skin problem like parasites that is making the hair either fall out on it's own or the dog is scratching and biting so much that it is making the hair come out. I suggest you get to vet so that they can find the problem and give something for the dog to kill off whatever is causing the problem. I have heard of dogs having problems with certain glands or lymph nodes where it causes problems with the dogs skin, I'm not totally sure about the lymph nodes. A buddy of mine had a problem with his Boxer where some glands or the nodes were causing problems with the dogs skin and the dog would lose hair from the problem and it's biting those areas.