Why?

After the slap, Derek had made it home on autopilot. Barely remembering last minute that he had snuck out, and sneaked back in, making sure that his mother did not know that he had ever gone out at all. 

But now he had just seen her off, and all that he had been repressing in an effort to make sure that she did not realize that something was wrong, had come back to the surface.

Pacing around in his living room, Derek tried to process, a hand over his cheek where Emily's palm had been a few hours prior. She had slapped him. Emily Molson had slapped him. Now, Derek was no stranger to sudden violent outbursts. He had grown up around entitled rich people, who seemed to think that it was their right to be violent. Vases flying, slaps, kicks, screams, spoiled brats who did anything and everything when they were throwing tantrums. But having grown up around that, he had never in his life thought that somebody like Emily, who was normally so calm, would slap him.