The way I see it, we'll never know for sure, but we have a few immediate contenders if we want to speculate like assholes.

1) Marshall is a mildly inconvenient personality but overall good guy who has a personal relationship with a key, [for now] irreplaceable player (Cutler) that either doesn't work either on a personal level or a professional level. Something behind the scenes goes wrong that forces the team into a him or me moment, i.e. triggering the on/off/on bromance back to off.

2) Marshall wasn't buying in fast enough for PaceCo and was creating enough friction that the new management decided to set a tone and send a message by sending away a player who is beloved by fans, has a great game, contributes in the run game, plays with heart, is making a good amount of money, who is at the end quarter of his career. Basically a bad short term move, but not a damaging long term move. The team basically relies on the media to exploit Marshall's controversial reputation to split the fan-base and ultimately quell any fan uprisings.

3) Marshall is an abusive person behind the scenes. Like many abusive people he puts on the best face possible in public. Perhaps like many abusive people, he organizes situations to make himself look good at the expense of the abusee in the eyes of the people around, to discredit the abusee. Marshall thereby relies on people with no experience with abusive personalities to carry his torch and down-vote anything that moves.

4) Ryan Pace never intended to keep Brandon Marshall. Pace knows more about White than he is letting on, and Fox and Pace agree that Marshall is not the right player for the offense that they want to build. Kevin White is literally Julio Jones.

5) Its none of these, and the whole world is just run by accountants and lawyers.