I believe there are several contributing factors to this war.
They are:
1- D'Amico feud
This feud with the D'amico's was not by design or planned but it's implications and how it was handled made the Rizzuto clan look vulnerable. Other OC groups realized that if a small clan from Granby can cause so much unrest with all the kidnappings and ransoms then imagine how much damage larger groups can inflict.
2- Vito Rizzuto's arrest
These criminal groups have the same laws as that of the jungle, they can spot weakness in a herd and strike on those that are vulnerable. With Vito in prison they felt this was their chance to mount an attack. I am of the opinion that this war began after Vito's arrest.
3- Disgruntled drug dealers and old vendettas
The Rizzuto takeover in the late 70's was hostile. Many from the Cotroni clan were not pleased by this takeover. This set back indviduals like DiMaulo,Cotroni's,Gallo,Arcuri's and others who felt they deserved a bigger share of the pie. Drug dealers like Piccirilli & DeVito and the likes were not happy with the new administration. The Violi sons were also plotting their revenge.
4- Montagna's deportation (2009)
This brought the war at a higher level. It gave birth to the coalition of rivals against the Rizzuto clan. More high level targets were being killed.This coalition broke up and it gave Rizzuto an opportunity to mount a counter attack.
5- Rise of the Ndrangheta
In the early 2000's the Ndrangheta had risen to be one of the most powerful OC organization. Ontario benefited from this rise in power and it limited Rizzuto expansion into Ontario.
This expansion wave to Ontario has reversed course to the point that the Ndrangheta is supporting rival groups in Quebec.