Minami is the leader of the second most influential clique in class 2F, and holds a grudge towards Yui, who she was friendly with in their first high school year, for being in the superior clique. She volunteered to be event organizer for the cultural festival in order to promote her worth without the actual ability to do such job, and thus requested help from the service club.
As such, Yukino took up the role of assistant event organizer and claimed the spotlight in making the cultural festival a successful one, which in turn made Minami depressed about her own uselessness and refused to host the closing ceremony. However, as Hachiman's fierce scolding has successfully turned himself into the scapegoat and shifted the blame away from Minami, she attended the closing ceremony and ended the cultural festival on a high note.

(Source: Wikipedia)