^Apparently, it's for studies though who really knows.
From her posts on G+, she was probably insecure on being the center and ace of M and wasn't feeling like she deserved it. Maybe Acchan's graduation affected her as well since she was the reason she joined NMB too.
She was young so she had time to grow and develop into a wonderful center but now, it's better to hope she's happier with this decision.