This is entirely speculative, but I wonder if the Perkins signing had something to do with this. My understanding is that the Cavs used their last roster spot on him, James Jones has been playing too well to get cut and dumping one of their other veterans like Marion, Miller to make room for him at this point in the season was something that couldn't be done. It always kind of seemed like Allen really wanted to keep playing with Lebron.
All that being said, It is a little surprising he wouldn't just retire at this point. It seems like coming back to the NBA after a year off at his age would carry a lot of injury risks to say nothing of increased chance he is no longer effective.