Answer: She realizes she hurt a lot of people, when Miranda told Andy she was just like her.
In a limo, after the announcement that Jacqueline is the new E.I.C., Miranda explains to Andy why she did it, and then tells Andy there is a lot of Miranda in her. Andy disagrees with that, saying she never did what Miranda did, and then Miranda says she did the exact same thing to Emily. Andy realizes that Miranda is right, and walks out of the limo when it stops, quitting her job. Miranda tries calling, but Andy just throws her phone into a fountain.