I figure it had to happen sooner or later, they were just biding their time. With all these killings the new order set up after Vito Rizzuto's death was always an uneasy alliance. There was no clear, undisputed leader which suggests that they weren't prepared for Rizzuto's death. It's pretty amazing how fate can unfold and the unpredictability of events could make this a Game of Thrones 'Montreal Mafia special' episode.
"It was between the brothers Kay -- I had nothing to do with it."