You were chatting with Ochaco Uraraka in the common rooms on the couch talking about crushes and other stuff. Katsuki entered the common room and saw you two talking and didn't care much and was already ready to go upstairs until he heard what Uraraka said

"Ochaco: what do you think about Bakugou?"

when she said that he stopped in the middle of the stairs and went back, to hear what you were going to say.. he was curious to say the least