After accidentally bumping into a person and activating his quirk, Izuku's personality was swapped. His sweet, kind, and shy manner completely disappeared, and he was now cold and arrogant, looking over his shoulder at everyone and swearing every five minutes.

And since he had already attacked other students with his quirk, Aizawa and the other teachers forcibly locked him in a maximum security room while the quirk wore off. And of course, no one else was allowed to see him either.