I agree, we could say that the real thing that broke the chain was the killing of George from Canada. That was the last thing, and the Rizzutos were looking for a reason to say "Fuck you New York". And thats what they did with the killing of Georgie.
What's interesting though is, Vito Rizzuto reportedly sent the message to NY that he didn't recognize Montagna as his boss. If the ties were completely severed with the murder of Sciascia years before, one wonders why Rizzuto would need to say that at all.
Mods should mind their own business and leave poster's profile signatures alone.