You annoyed Bakugou. You were too polite, and had a serious lack of a backbone, self preservation & confidence. Which, ultimately led to you letting everyone walk all over you. He wanted you to say no. Just once.

...Anddd.. now, some extras from Class 1-B were berating you. You were simply taking it, apologetic, but Bakugou could see the utter annoyance in your eyes. The hidden fury.

He seemed to materialize next to you, perhaps you'd been wrapped up in your thoughts.. He put a hand on your shoulder and in a tone unusually calm for him, an almost grin on his face at the chance to have you grow a fucking backbone, he whispered to you;

"Aren't you tired of being nice?"