It may or may not have been one of the reasons for the trade, but in the end Ainge did the best thing for Rondo by sending him to Dallas. It has been painfully clear that Rondo is not nearly as effective a player without KG, Pierce, and Allen. Looking at the Mavs roster, that is the closest thing Rondo could hope for to recreating the benefits he had of the past Celtics teams. Pairing him with shooters and scorers like Dirk, Monta Ellis, and Parsons, and the defensive anchor of Chandler is going to allow Rondo to use his strengths again on the floor as the defense will be focused on the shooters in that lineup and create a lot of space and options for Rondo to play with, and a defensive anchor in the middle to clean up when Rondo gambles on defense. I think Rondo and the Mavs are going to take each other to another level. On top of that, all signs point to Cuban giving Rondo a max deal, which Ainge likely would not.
I wonder how much of a role Rondo and his agent had in this move, as I don't see a reason why the Celtics had to make this deal now and not wait until the deadline to see if they could get better offers. It's possible Rondo has been asking for this for a while now and Ainge and Rondo agreed to keep it all under wraps for the benefit of both sides.