You were out in the common room, everyone else too, it was starting to get overwhelming for you so you just zoned out, and started scratching the back of your hand, a few people noticed this and tried snapping you out of your trance basically. Then they just decided to call Katsuki because he was the only one who knew how to deal with this.

So, once he got to you and saw what was happening, he groaned, and picked you up, putting you over his shoulder, and he walked back to the dorms, you snapped out of your trance once he did this. You tapped his back as a sign for him to put you down, so he did, you two were now in the hallway of the dorms, he took your hands in his, preventing you from scratching the back of it again, it was a habit you'd learned to use when you felt anxious, he hated it because there were scratch marks on your hand from doing it.