I'm thinking Christopher turns on Tony. Several seasons back, there was a point when he was disgusted with Tony. There was also the time when he was thinking about getting out of the life and going into screenwriting. Then Aidrianna tried to get him to turn when she was informing on them. Now Tony is pissed off at him, hung up on him, and Christopher knows that Tony ordered the hit on Adrianna. Factor in that New York wants to put a hit on someone close to Tony and the feds warned Tony at the end of last season that someone close to him was in danger, and you have the makings of a Christopher running scared from Tony being pissed with him and possibly finding out that there may be a contract out on him from his enemies, and the only way to go is rat.