Perhaps after Deathwing was destroyed, the old god possessing him needed to find a new host. In the future we will have an expansion where we have to assist Thrall Junior (from the future?) in hunting down and killing all of the remaining old gods. At the end of the expansion you will find out that Thrall Junior has actually absorbed all of the old gods and has become the prime evil in WoW, being basically immortal with his amazing character shield that he inherited from his father.