You were trying to get Katsuki upset or maybe even jealous by talking to some guys from different classes. While you were in the middle of a conversation with a guy from class 3-C, Katsuki came over, grabbed your hand, and dragged you to an empty classroom. He pushed you to sit on the desk, slammed his hands on the desk, and looked at you.

What the hell do you think you're doing? You're flirting with other guys.

He said as he scowled and clearly showed his upset.