The breakup should happen after Dean Ambrose loses the US Title because that should be the moment when he snaps on the other members of the team. It would make him feel like an inferior part of the team when he finally loses his gold. Ambrose is a key figure in the breakup because he's likely the one that will be the heel out of the group and right now the US Title is on him to keep his ego strong. Then again, WWE barely has him defend the title so who really knows why the title is on him? For the breakup to really work, Ambrose has to be the cause of it. He has to think he's the weak link or the loser in the group and that's why he will cost Reigns a big match or attack him because he's so mad about losing his title. The gold may not be defended that much, but it's a key part in The Shield's story in terms of how Ambrose is going to be booked in the coming months.