Could be a number of things. He could have heard about a contract out on him, he could have heard a tape of discussions where his name was brought up (like Sammy Gravano-who wouldn't have snitched on John Gotti if he hadn't heard the conversation about him on tape)...could be a number of reasons why he did that. Even though he is still a BOSS until death, some lower ranking capo could have gone against the rules and put a hit out on him...OR, a Capo or somebody under him could have consulted with the other families and recieved permission to hit the boss.
SO MANY THINGS could be behind this guy snitching. Or, it could just be a selfish act to try to get himself out of prison. Who knows? La Cosa Nostra isn't what it used to be.