I've read a few comments out there trying to make it out that Sebastien is some kind of disloyal jackass for wanting to join Apple's Core OS team as VP. That is NOT the case. His loyalty and job history proves otherwise. Just have a quick look at his LinkedIn profile and you will see that Sebastien has been with QNX since 1998, and has been loyal throughout the BlackBerry acquisition since 2010. While I'm sure that we all agree that a contract is a contract and should be abide by. But that doesn't mean other factors can't come into play that change the context of the contract.
So why would he want to change teams?
What I find most revealing and important about the case is the part about Sebastien's interaction with CEO John Chen as one of the reasons for him wanting to leave. Sebastien is a big visionary thinker, and John Chen is set on making BlackBerry into a more niche company, focusing more on the business aspects. BlackBerry's pockets are shallow right now, and Apple's pockets are deep with cash. It takes resources to implement big ideas into physical manifestations.
From the case file:
 During the fall of 2013, BlackBerry underwent some difficult times. In November 2013, John Chen (“Chen”) became BlackBerry’s new Chief Executive Officer.
 Marineau-Mes continued to play a central role at BlackBerry during these times, and had discussions with Chen about his future that were not all that satisfactory to Marineau-Mes. It was discussed, amongst other things, that Marineau-Mes’ role might ultimately be narrower in scope than originally contemplated.
What I personally know of Sebastien Marineau from the times we've met, is that he is a stand up guy and a class act. Every BlackBerry event we have attended he has always been right there willing to make time for an interview, or even hang out and chat about whatever. Take note of the picture above with Sebastien wearing the BlackBerryOS.com t-shirt we gave him during a video interview at the BlackBerry Jam 2012 dev conference in San Jose. He insisted that he was going to wear the shirt during our interview, and just started stripping off his work shirt right in front of us to put on the BBOS tee. Def not what we were expecting! But we were like, "hell yeah!"
Anyhow, Sebastien has proven his worth to BlackBerry with his hard work and dedication. And if Chen wants to change things around, we need to expect the executive team to change as well. But this ain't over yet, anything can happen within the coming months left for Sebastien at BlackBerry.