I choose to believe that he is struggling with the expansiveness of the world he has created. I think the "spin off" help him think through the back stories and hence the motivations of the various characters. It is his world and he wants it to be internally consistent. The HBO series getting ahead of the books would be very de-motivating. Does he make the series wrong? Does he write a book that just re-hashes the series? Does he pile on with the critics of the series? I think he wants to make it something new without burning any bridges with the producers of the HBO series. Tough job trying to thread that needle.
