The UFC already offered him a contract at 205 but he turned it down because he is making better money in Ray Sefo's new promotion (which i think will go under soon because there is no way they can pay their fighters what they do and stay above water). I think he is probably looking for some easy wins at 205 to build his record so he can negotiate a better deal with the UFC when his current WSOF contract (he has two or three more fights) is up.
He has looked really sharp at 205. His last opponent, Linderman, is no chump and Rumble really put it on him. Training with Rashad and Reem everyday is clearly paying off.