Originally Posted by Rochard

Well, you said it all right there. He was "not cooperating".

Police roll into a "domestic" and when get on the scene and start asking question, they have someone who is "not cooperating". It doesn't matter if he was part of the disturbance, a witness, or someone who just happened by - He was on the scene and a potential suspect, and when police tried to question him he failed to cooperate. At that point police believed he was a suspect and that he was involved, and decided to detain him until the situation was figured out. He resisted.

At that point, he did in fact break the law - resisting arrest.