I really does not matter why he did anything, particularly as you cannot know a person’s motivation – that is literally impossible until we learn to read minds. What does matter is his actual actions. That is what you need to find fault with.
So far, it appears that you have found fault with his actions solely because it does not promote partisan hackery. I find insurmountable problems with that position.
The actions lead me to believe that he's out for himself...As in a politician who will do just about anything to raise his stock.
As for the hackery, he signed up to be a republican, a party man if you will...
You didn't need to have Rove or Carville on hand to know that what he did was damaging his own party's candidate at the worst possible time....As a politician -and make no mistake about it, Christie is a politician first- you don't do such things without first making some sort of political calculation as it relates to yourself.
The whole "he did the right thing for his constituents" thing is an Olympic pool full of hogwash, no matter which way you want to slice it.