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?