I for one thunk problems rooted way before that in 2000 as Vito tried to push and consolidate his power becoming a sort of boss of all bosses by taking over Ontario.

It didn't take much for the siderno group to set him straight by killing pane pinto, his main man in Toronto. That push into Ontario might have given the siderno boys Extra reasons to find unhappy campers in Montreal and unite to push the Rizzuto family out. The bigger you are, the larger the target on your back is