Well, like I said, it was her own way of atoning. She probably did it soon after the incident.
Apparently Miichan valued her hair. It was part of her pride. And now that she's lost her pride, she has to think about what she's done.
Like I said, it had nothing to do with the management and it's quite silly to assume it did. Shockingly, idols actually can act of their own free will!