It sounds pretty logical to me that Montagna had a hand in organizing those killings. But the Violi brothers were very likely involved too, and possibly 'Ndrangheta elements.

The Bonannos probably gave him permission to become the new caporegime in Montreal. That's why Rizzuto said that the wouldn't take orders from him. And likely also because Rizzuto thought he was Montagna's senior, which he was. And therefore Montagna organized his murder, after forming allegiances with the Ontario families and establishing a powerbase.

But in the end, Montreal mafiosi decided that Montreal should be for themselves and they took him out. They didn't want another tyrant. Montagna was probably considered an outsider and he apparantly had made some enemies, respected enough to order his "disappearance".

The exact reason why Montagna was killed remains unknown and at this stage we can only speculate. But it sounds plausible to me that he had started to act too much like a top-dog.