Because he used to be the guy worshiped by everyone around him. He used to be the center of attention. But that's not the case anymore. NOBODY in his new class treats him the way people in his old class treated him. He's not the superstar to any of them. He's just the loud agro guy.
And he WAS kicked in the teeth. When he lost to Deku. It's completely changed his character. His personality APPEARS the same, but it's not. He's all bark and no bite now. And he hasn't bullied anyone since then.