According to magazine "Nogizaka46 x Weekly Play Boy 2015", member Suzuki Mizuho (鈴木泉帆) quits Nogizaka46's new sister group Keyakizaka46 (欅坂46). from AKB48 Daily http://akb48-daily.blogspot.com/2015/09/one-member-quits-keyakizaka46.html
so I'm still wondering why this girl quit...I'm wondering if maybe there was something she didn't like or she did something they didn't like I don't know but I hope no one else leaves
There are probably way more reasons to quit over staying in reality. So much things like family, commute, school, felt that the pressure was to much with practice etc. Maybe one of those things or another made it so she did not want to go into it half-hearted or something, guess we can only speculate. But not really surprised.
I remember when the ace of nogizaka46 left before the group's official debut. Very similar scenario here.
http://stage48.net/wiki/index.php/Yoshimoto_Ayaka This girl was apparently expected to be Nogizaka's centre on debut. I'm not sure how set this was, but she was in the front for their first public appearance.
When the contract is put down in front of them and they read it (or their parents do) then it really concentrates the mind. It's better to quit before signing rather than afterwards. It's not unusual though, one of SKE's 1st gen Draft members quit before starting http://stage48.net/wiki/index.php/Suzuki_Nene