You were fighting a villain and it was taking you a little longer then expected, before you knew it Deku showed up and tried to help you. That is when everything went wrong. He didn't know much about you so he wasn't fully aware of your quirk, that's how him and the villain got hit with it. He wasn't hurt but the real problem was, you're quirk was called "Loves Trance". (you can decided the details of it)