as others have said, its a dominance thing, have 3 of last years hobs all caged together, and they seem to have worn themselves out for this year, but another 3 of this years hobs, which are caged seperately, two of them have the scaby back, with "boston" being the biggest and therefore more dominant, I personaly dont think yours looks that bad, and would have left them togther, the nights are getting longer, so they will soon give it up, try putting either some SUDACREAM(babies bum cream) or teatree oil on the scabs