don't think Dumbledore meant to humiliate Snape deliberately,

This is Dumbledore's problem- his incredible lack of empathy. He is a person who is easily dazzled, and he does believe that he is do the right thing. But in this scene, he is clearly enjoying Snape's discomfortire, thinking that Severus "will get over it" to paraphrase what he says to Harry. Working backgwards from OOTP,you can see the structural strain ebgin right there. And by the way he says "You!" in GoF, Severus has not been told one damn thing.