He did whip his head around but I sensed no aggression.
I would recommend educating yourself on basic canine body language. A head flip is a measured and appropriate warning by a dog to stop what you're doing. The dog likely bit you because you did not listen to him when he did the head flip. The fact that he did not break your skin is also very deliberate. Next time you grab the collar, it could be worse.
"Fault" may not be the right word here. The shelter may or may not have known this dog had handling sensitivities. Or maybe you came off too domineering with a dog that did not yet trust you. Who knows. Fact is, this is the dog you have in front of you. I would not use force on a dog like this.
Also, "pit bull" really doesn't have any credence these days. I say this as a person who has seen hundreds of them and love them. Traditionally, they should show absolutely no aggression towards people, even in the worst scenarios. But pit bulls today are overbred, poorly bred, cross bred, to the point where the standard temperament cannot be predictably found in a shelter dog.