Somehow I feel that this whole Miki leaving MM mess was planned.
She isn't very close to most of the current MM members as we know it, the group isnt getting a major boost in sales either, and with her being the new leader, im guessing she just doesnt want to/couldnt handle the burden of babysitting the other girls as well as Yossie can when she was still there.
She says the reason why she is leaving was because she violated the leadership role... saying how she has a heavy responsibility towards guiding other members. If she REALLY cared about this so much, she wouldnt have considered going out with a guy, let alone spending 3 consecutive days with him. On top of that, she wouldve known with all the recent scandals of Kago and Tsuji that the paparazzi are going to be hot of their tails, hunting for any dirt that can find.