Graham had an "episode" during training camp, who's to say that Graham was not brought back to the team to help him transition into retirement because maybe he was having some issues in his personal life.
Hard for me to envision an NFL team burning a precious roster spot for this purpose, especially on a player that, while he was great here, he kind of burned bridges on the way out and wasn’t really thought of as “One of our own” anymore in every sense of the phrase.