Nothing actually quite surprising. He may have been pressured into this some time ago after being confronted with something we are not aware of. Quite a few prominent mob members turned out to be informants. The most famous was Jimmy the Weasel Fratianno who eventually turned into state's witness for one reason; it was found there was a contract out for him for being an informant.
James Whitey Bulger, who Jack Nicholson's character in the The Departed was modeled after, was a longtime informant.