Sounds like she already answered your question
pwBPD tend to exhibit the same behaviors and coping mechanisms as a child. And according to her, that is what she is doing!
I've actually found it helps me to rationalize and have more patience with the chaos if I think of my fiancé as someone who did not learn adult coping mechanisms, and I adjust my behavior as if I was a parent.
As for specifically why she does this - well, why do teenagers do this? It may be the same reason. She may just want to express her independence from you.
