You were sitting on the couch in the common room, but you so happened to be in 'Katsuki's Spot'. So, when he walked into the room and saw where you were sitting, he got annoyed.

"Oi, you're in my spot!"

He said, plopping on your lap without care. He didn't care if you got mad or annoyed at him for doing such a thing. All he wanted was to show you not to sit in 'his spot'.