It's not 100% clear why Bron Breakker kicked the Vision leader, Seth Rollins, out of the group, but one reason could be that WWE may have found their next Roman Reigns.
There has been a lot of talk about Rollins' ousting from the Vision. He appears to be dealing with an injury of some kind, and that could be playing a part in the sudden pivot.
Additionally, the Wrestling Observer Radio may have pointed out another reason for the betrayal. WWE is seemingly moving forward with pushing Breakker to the moon as their next Reigns. The move “was about Bron Breakker” more so than Rollins and making Breakker “the next Roman Reigns.”
The Vision turning on Rollins was always the plan, but it appears to have been expedited. It puts Rollins' penciled-in match against Reigns at WrestleMania 42 in flux.
“Seth Rollins was tentatively scheduled to wrestle Roman Reigns at WrestleMania,” the Wrestling Observer Radio said (via Cultaholic Wrestling). “I don’t know this, but I have been told enough things to where I expected there to be a turn on Seth Rollins at some point. The whole thing with this faction was not about Seth Rollins, it was about Bron Breakker and making Bron Breakker the next Roman Reigns…I doubt it was going to be this early.”
Where do Roman Reigns, Seth Rollins, and Bron Breakker stand with WWE?

The date of Reigns and Rollins' next clash appears to be up in the air. They have a long history, and WWE will surely book their rubber match in the future.
For now, they are likely waiting on Rollins' injury status. If he is out for the long haul, the Vision may move forward with Breakker, Bronson Reed, and Paul Heyman before Rollins eventually returns.
Reigns is now directionless if Rollins misses time. He is feuding with the Vision, but the ultimate endgame appeared to be a match against Rollins. Now, it is unclear where his story goes.
Breakker has always been highly regarded by WWE. He is already a two-time NXT Champion and Intercontinental Champion. It's only a matter of time before he wins a world championship. Perhaps that will come sooner now that Rollins may be out of the picture.