Ainge gave Rondo the chance to be the man, to become a 16+/10/5 pg and the leader of the team. Ainge hoped that Rondo would step up his game and cause the young ones to play better as well, especially in the horrible Eastern Conference.
Problem: Rondo isn't a scorer and add to that he is a poor shooter.
Problem: Rondo became a stat stuffer, looking for the triple double every night.
Problem: The team, and the young players were not showing great improvement as evidenced by the W-L record.
In other words, the GM and ownership determined that, unlike Pierce, Rondo was not the piece to build around.
Solution 1: Trade the young players i.e., Sully, KO, Bradley to have a veteran crew for Rondo . That did not work in the summers past.
Solution 2: Trade Rondo and other veterans and commence full blown rebuild.
I personally think the package from Dallas is lacking, but I do not know what else was being offered.