It's because it's an example, really you could use either, but the daughter is trying to make an image in her mother's mind, to describe how she (her mother) is behaving, and why that behaviour is causing problems. I'd be more likely to say, "you're" (causing problems) when it's just a case or either me or you - it's not me, so it's you; you're causing problems!! It's making an example, to express a point. "You're the one who's acting as though she is that woman/ she is a woman who is running around trying to be...............etc" I hope that helps.

The writer of this is not writing correctly. People often make mistakes because they don't know proper grammer. This author switched from "second person" to "third person". It sounds like when talking to her mother, she should have correctly said "You're the one always acting like someone who's running for mayor of Stepford."

You're THE ONE always acting like SHE's running for mayor of Stepford. the one = she

This is very simple and very common. The meaning is "Your acting like [a girl] who's running for mayor of Stepford" and what is the personal pronoun for "a girl" - she!

It is clear that their is a third woman involved. Neither the mom nor this daughter are being talked about with "she's"