I'm thinking either Cullotta told other Outfit guys about Spilotro's ambition to take out the midwest bosses, or Spilotro himself babbled it to too many people and it got back to Chicago. He told Cullotta he wanted to be Lucky Luciano and for Lefty to be his Meyer Lansky. These were the ravings of a coked up lunatic. DeNiro's reaction in the movie is perfect when Pesci hints he wants to take over the Outfit. Just stares at him knowing he'd lost his mind. In the book, Cullotta says he had no choice but to pretend he was going along with it because the second Tony detected he wasn't, Tony would kill him. You have to wonder how many other soldiers and associates Spilotro strung along with this deranged idea. With him talking this way and people knowing about it, it would make sense that the bosses wanted to make one serious example out of him.

"...the successful annihilation of organized crime's subculture in America would rock the 'legitimate' world's foundation, which would ultimately force fundamental social changes and redistributions of wealth and power in this country. Meyer Lansky's dream was to bond the two worlds together so that one could not survive without the other." - Dan E. Moldea