Bungie posted about it. (Destiny)

by Schooly D, TSD Gaming Condo, TX, Wednesday, April 16, 2014, 00:23 (3884 days ago) @ Cody Miller

6. Marty decides to leave, knowing there are any number of game studios ready to pick him up and pay him mad cash

I think this jibes with the disparate tones of Bungie's news post and Marty's tweet (including the "without cause" qualifier seems like something resembling bitterness).

Yes, which is why your #6 doesn't make sense.

This is complete speculation, but it is also possible he was seen as a liability in the wake of Brendan Eich being ousted from Mozilla over his political views. Marty IS known as the most conservative one at Bungie after all, and pretty far to the right.

There's no reason to dismiss #6 because the tweet was bitter. We don't know the amounts of money being thrown around -- what Marty was making, what Marty was expecting, or what Marty was offered. If BNG-ACTV said "thanks for the music, by the way we're cutting your salary 25%" you'd be in a situation where both (1) Marty leaves voluntarily, knowing he can get more money elsewhere, and (2) Marty is bitter about it.