There are only two possibilities:

1) The hit comes from New York. What NE1020 suggests is plausible. Or, New York kills Tony because Carlo is going to testify against Tony (Tony will almost definitely be convicted this time) and they worry Tony will in turn become a rat and testify against New York. This would be a preemptive stike to protect New York. Tony just knows too much.

2) The hit comes from people beneath him in his only family. He almost got killed before by Uncle Junior, Richie Aprile, Paulie Walnuts, that tall guy that Silvio killed with a piano wire, etc. He has a history of discontented people in his crews that consider killing him.