A BPD sufferer has an unstable sense of self, meaning who they are to themselves changes, sometimes minute to minute, and that has nothing to do with you. Plus, sufferers are impulsive, and if they do something to soothe, meaning use a maladaptive coping mechanism like drugs, booze, sex, self mutilation, whatever, and then feel guilty or shameful about it, another maladaptive coping mechanism is to project that on you, make you the scumbag and the scapegoat, again, it has nothing to do with you.
Then there's the fear of abandonment, the core of the disorder. If it appears to a sufferer that you have or are about to abandon them, they will look for other attachments, because abandonment means death, and will blame themselves, shame at the core of the disorder also, be uncomfortable with that shame, project it on you, and again make you a scumbag to justify their actions and allow them to feel better.
Whew! Check out this article if you haven't and see if you relate:
https://bpdfamily.com/content/how-borderline-relationship-evolves BTW, we are all impulsive, project and use maladaptive coping mechanisms at times, and a borderline is living in their own personal hell, so it's just maladaptive on steroids.