Im not a montreal mob expert and im not that familiar with its history but my theory is that the rizutto clan just got weak and wasnt able to provide the same protection they used to to the footsoldiers and associates. So a bunch of influential guys get together and decide that they need a new rejuvinated leadership, not one thats slowly dieing. They get montagna involved because he has links to ny which cant hurt and also because he got a good racket, they probably also reached out to some calabrians as well. Only thing is after its done montagna decides like sonny said that he wants to be the boss or the authority of the group and ends up dead.

However i think that the whole haitians, street gangs, and firebombing of the cafe was a sort of separate drug trafficking dispute all together. Im sort of tempted to throw in the assassinatin of rizutto jr in that category aswell. This might have given the dissident faction within the clan a clear signal that now was the right time to move in and start taking people out themselves.