He also simply wanted more money. He thought he was the STAR of CM, and that the show wouldn't survive without him, so that contract negotiations were in his court. Notice he was missing in the first 2-3 episodes. The writers literally had to write around him because he didn't show up to work as scheduled.
Filming ends in late April. It starts shooting again at the beginning of August. They had 3 months to re-negotiate his contract and the "creative differences" he says it was about. The writers start writing the first episode at the end of JUNE. So the first few episodes were already written, along with how much violence in them, when he didn't show up for filming in August.
Last spring, Morgan/Shemar Moore's contract was re-signed right away in MAY.
Hotch/Thomas Gibson, who also wanted way more money, his contract took till July, exactly one month BEFORE shooting to hammer out. When he was at the Monte Carlo Festival, he said the studio execs' side had stopped talking to his agent completely. He wasn't even sure if they WERE still negotiating.

The writers said they were writing alternate stories in case he didn't come back.
This is all documented in last year's posts here - so I know the accuracy of the time lines for the 2 actors, when the writers start writing and when they start filming I got my info from their own Twitter feeds (Twitter pics when they started filming,) and any news stories I could find at the time. They all corroborated the same details.
This isn't the first series Mandy walked out on. He ALSO left
Chicago Hope. And it doesn't take till the second or third episode into filming to know that CM is going to remain violent. The producers would have told him way before then that their "creative vision" of the show will REMAIN at that level, and asking to read the episodes at the end of July would have show it.
While they were re-negotiating Thomas Gibson's contract last summer, even HE made a snarky remark about Mandy leaving TWO shows that he was on WITH Mandy, and that he knows it was about more than what Mandy says it was. It was the first time I have ever seen or read him saying something bad about someone else.

But, he didn't want to be lumped in with Mandy if they couldn't re-negotiate his contract and he ended up leaving the show. He really wanted it on record that he wanted to stay on the show.
As for coming back to Broadway, that is inaccurate - although he may have hoped to do that too, once free. Mandy was on CM from 2005-2007. He hasn't been on Broadway since 2003. This winter was the first time he's been back.
http://www.ibdb.com/person.php?id=55659