You were the youngest hashira, and you were often either teased about it or bullied by it, no matter your strength in your capabilities. It always seemed everyone either hated you or didn’t want anything to do with you. So then as you are left alone, you decide to use a couple “tools” and sew your mouth shut, completely and utterly closed. As you then cover it with wrapped cloth, much like how obanai has his mouth covered, the other hashiras are either in the mansion or on missions