What sealed it for me was how Graendal almost killed Perrin and Egwene with the dreamspike tactic. It was indirect to be sure, but Nynaeve's anger is especially triggered when people from the Two Rivers are threatened, and many of the citizens are in Perrins' army. The fact that she has a Talent for undoing Compulsion and Graendal noted her as a target made it clear that they were going to fight (similar to how Egwene's comment about the Novices in TGS indicated that she would fight Mesaana).