That's the question, isn't it?
I'm guessing that the actual firing was a matter of a period of discussion. Somewhere I read that Tagu blogged "Ayaka will save me" which tells me she didn't go willingly.
I keep wondering what the girls were thinking when they took that pic, especially Hamachan as they were KSS mates from the same generation.
Ayapan mentioned that in her first post about it right after the announcement was made. It was in Taguchi's last blog post, but it was part where she was talking about all the members. Something to the effect of "Ayapan will protect me. Nomu is my sun. Hama-chan is a special generation mate. Sako is my moral support. Rei is already too cute." (If someone can make that more accurate, please, please correct it.)
I would love to see someone translate Taguchi's last blog post. There seem to be some things that could be taken as her knowing her time as an idol is over. (It seems one line is "Sometimes I make even enemies that I do not have to make" and another says "If you walk a normal life, you will not have much love pouring from so many people!")