He knew he had a job with Dan Quinn lined up, which meant a quarterback upgrade, so he exercised his leverage and forced the Browns' hand.
There are many ways to look at that move. If you're a Browns fan, you probably feel deceived. If you're a fellow NFL coach, you might applaud him for maximizing value in a fickle business that leaves good coaches on the street every year when they don't win enough.
If you're a Browns exec, you might be feeling regret -- as in, why did I send those text messages to the sidelines suggesting Shanahan run certain play calls? (We don't know all the details/semantics of the Cleveland.com report about Shanahan's displeasure with non-coaching staffers mettling in his playcalling business, but the NFL's investigation on the matter signals a problem).
It's also worth wondering why the Browns weren't stronger in their contract talks with Shanahan. He clearly didn't want to be here, but why let a good coach with two years left on his deal walk out? If he stayed, he would have worked just as hard becuse his offensive rep is on the line. No reason to hold coaches hostage, but the Browns were well within their right to request he do his job for another year.
Now, if you're Mike Pettine, you have every reason to feel a bit burned. Pettine hired this guy when Baltimore didn't (it hired Gary Kubiak instead), never meddled in his meetings -- in fact, from what I'm told, Pettine gives total autonomy to the OC on offensive matters unless executive decisions had to be made, such as quarterback changes. Watching that guy leave for, at least on paper, a lateral move must be disheartening. Shanahan's offense scored four touchdowns in the season's last five games.
Whatever you read about Shanahan's reasons for the departure, the primary one is this: He chose Matt Ryan over Johnny Manziel/Brian Hoyer/draft pick. It's a clear indictment of the Browns' quarterback situation, and the team's place in the 2015 NFL landscape. It's not lost on Browns coaches that the NFC West is on the 2015 schedule. The Ravens, Steelers and Bengals aren't dropping off. Two of those teams have top-shelf quarterbacks, and the other has a steady option in Andy Dalton.
From his days as Houston's quarterbacks coach to Washington's OC to Cleveland, Shanahan's never had true stability at quarterback. He has it now.
He needed a clunky exit from Cleveland to make it happen.