I think it’s a combination of several factors:
If you have BPD, you desperately need someone and that makes you mad. It’s upsetting to need someone that bad, and to depend on that person to feel complete.
Even when you need that person, you can’t trust her/him. You may not notice that the lack of trust comes from your own distorted perception. In your mind, that person is not trustworthy and that’s upsetting. The same way that you split as a child to be able to live with your parent, that happens with your partner (off your radar).
pwBPD (person with Borderline personality disorder) have control issues. They feel out-of-control so often, that they crave controlling others. When a partner is not controllable, they feel invalidated, hurt, alone, inadequate, etc. That creates a lot of resentment that will explode at some point with the intensity that characterizes the disorder.
A dysregulated brain like the one of a pwBPD goes up into the fight/flight very fast and very high. When that happens, the survival strategies will do whatever to keep you safe. That means that the danger you perceive is enormous (abandonment for example) and you’ll defend yourself no matter what. You may react in a similar way that an animal reacts when it feels cornered. In those moments, it’ll be a matter of life or death.
When a pwBPD gets dysregulated, they lose the capacity to see a partner. They see an enemy that is threatening their safety.