I think you may have read it wrong, Activision recommended his termination, because Activision replaced the original trailer score with their own stuff. Then when O'Donnell was frustrated about the whole thing, Activision basically said to Bungie to drop him because "he could constitute a breach of the parties contracts... What it sounds like is that Activision basically "corrupted" Bungie by going behind their musical director and then firing him because he got mad about not being told his score for the trailer would be used on a last minute ditch effort.