Otsuka Aina was originally a finalist in the Morning Musume 9th generation auditions back in 2011 before being added to Hello! Project by means of the Kenshuusei. Juice=Juice was formed in February 2013 with six members (Miyamoto Karin, Takagi Sayuki, Uemura Akari, Otsuka Aina, Miyazaki Yuka, and Kanazawa Tomoko), releasing their first two indies singles later in the year, and scheduled to have their major debut in July.
The news notice regarding her withdrawal from the company was as follows:
As always, thank you very much for supporting Juice=Juice.
We have sat down with Otsuka Aina and her family several times in negotiations with regard to her contract, but regrettably we must inform that it has been without reaching a mutual agreement. And so, in order to respect her family's wishes in declining the terms of the contract, as of July 5th Otsuka Aina has withdrawn from both Juice=Juice and Hello! Pro Kenshuusei.
Here on out, Juice=Juice will be continuing their activities with five members.
To all of the fans: we beg for your understanding and continuance in cheering them on.
Otsuka herself posted a blog entry regarding the matter, though it has since been deleted from the Juice=Juice blog:
Though there are probably those who already know…
As in the official announcement、
Otsuka Aina will、as of today、be withdrawing from Juice=Juice and the Hello! Pro Kenshuusei。
I really apologize that I’m not able to give a proper salutation in this way。
Everyone who’s supported me until now、
thank you very much up until now！
Well then、bye-chaーko (*´ `*)
Source: 01, 02, 03 (blog translation).
This makes me sad. :( I really liked her. It's unfortunate that it was due to the contract, presumably regarding Juice=Juice's major debut. There's really only speculation at this point as to whether Otsuka wished for this herself, but it does seem like it was her family's decision more than anything else.