It happens if she has managed to steal Violet from you at least once and is wearing her.The attack happens during fights after you free yourself from her grasp and are not wearing any clothes.
Got it working, thanks! Now, if only I could reliably repeat the scene where Nel and Violet fuck Evie together...
All scenes that are only available once in the story (and already found in the game) are available in the gallery, in the Dream World.
