Lots of different theories revolving around the concept of malice and heated arguments being the cause of Marty's departure, the fact that the nature of his leave was "termination without cause" means we may never know, at least until Marty himself knows...
"Termination without cause" usually means "You didn't do anything wrong but we just don't require your services anymore" Marty's employment wasn't a periodic contract based one either.
So does this mean that someone, somewhere within Bungie had planned and scheduled Marty's termination for after he had completed Destiny's initial soundtrack?
Honestly I can't be angry at them...because they're insane. Like, fine, the Destiny soundtrack is finished. Fine, Marty is inconvenient financially. Fine, he'd be able to find work elsewhere, no problem. But he must have been asking for a heck of a lot of money, or I don't know how anyone could have thought this was a good idea.