Reporters published direct quotes from several players. It's one thing to be in a coach's doghouse, but Montgomery was suddenly a pariah with his teammates, too. Already being a low usage, low value player in the final year of his contract, his a$$ was grass.
Just to be clear, it was indeed 100% verified post-game by the press that Montgomery was instructed to take a touchback in the end-zone. And that the rogue act was preceded by a visible temper tantrum he threw on the sideline after getting subbed out the previous possession.