She is mad, but she is not sure she is justified in being mad. She has some doubt, so she waits for you to justify her anger by apologizing. If you do, then she was right to be mad, and you proved it. If you cannot think of her reason for being mad, then she gets angry at herself for misinterpreting your intentions. OR, she goes to her friends who instantly, and thoughtlessly justify her anger, because that's what friends do. So after getting her dose of justification, she will expect that you do actually know what is bothering her, but you are too stubborn to admit it, out of fear she will get yet more angry. This angers her MORE, this is the point where the argument finally takes place, with her in the worst possible mindset.
How to avoid this:
...
umm
Don't let her talk to her friends, then maybe she won't get justification, and can listen to your side of the story objectively. Maybe.
