I can't imagine how it came to this. Clearly there was a falling out, but was it over money? Did Marty believe he wasn't getting paid enough for his work on Destiny? Maybe when they signed the deal with Activision he presumed he was getting a better deal than Bungie actually intended to offer.

But even so, it must've really escalated for Bungie to actually go and make the decision to fire him. I imagine the standard procedure in these disputes is for the aggrieved party to resign and come up with some hand waivey PR line about why and save face. Unless the Bungie execs really were just boneheads, it must've been something really serious for them to expose themselves to the PR blowback by firing him.

Other possible explanation is that Marty screwed up, but if that was the case he's risking a lot by going to court and having it all put out on public record.