earlier this year, he butted heads with coach rick carlisle and Dallas suspended him a game for arguing too much while on the bench. rondo has a reputation as headstrong guy, and as a player he's the type of guy that likes to run the show. He's a pass first point guard that creates opportunities for teammates to score by dribbling the ball very well.
However, in recent times, Rondo's shooting has been dreadful. He's never been a good jump shooter from the field, but that was OK in boston when he had 3 guys that could score like Pierce, Allen and KG playing around him. When the Big 3 were hitting shots, they created space so Rondo could still get points by attacking the hoop.
Mavs have some Good/Decent shooters (Nowitzki, Ellis is ok, Parsons is hurt but is generally pretty good), but the biggest problem is that rondo's free throw shooting has fallen off HARD. He usually shot around 60% from FT, which is poor from a guard but not bad enough to hack every possession. This season he's shooting under 40% from FT (around 45% since being traded to dallas). Shooting a low percent from free throw makes his drives to the hoop less effective, since teams will just hack him and make him shoot free throws that he will usually miss. With no shot and diminished drives to the hoop, teams will sag off him and guard his teammates while daring him to shoot. His drive and pass aren't as effective, and now he's looking like a liability on offense.
IMO, Carlisle not happy with how he's playing = not trusting him = butting heads. And carlisle is a vet coach that has coached vets to success (aka to a RING). But you got to buy into his system and play the role, and rondo just doesn't fit nor does he really seem to be trying to
Also, he admitted to not playing D very hard while he was still with the Celtics. C's weren't considered title contenders, but as supposedly the best player on that team after the big 3 left, that's not a good look from your franchise player. Also he admitted that the season before he's a free agent, like c'mon you're supposed to try harder your contract year