Instead - I said okay sure, but obviously my body language or facial expression made it clear I didn't want to.
This is something that I realized I needed to pay attention to and work on in myself. I had the same tendency and still sometimes catch myself.
As once removed said above, "no" can be a trigger, but bear in mind that if you're saying "yes" verbally but making it clear in tone, body language, or facial expression you are annoyed by the request, that can be a trigger as well. It can honestly feel like a game or a passive aggressiveness to even non's, so imagine that BPD sensitivity - especially when they are in a mode of *looking* for complaints - can exaggerate that.
Looking inside myself, yes... I was sending that non-verbal signal as a way to signal my annoyance. I understand why, but being honest with myself, I realized it's not a mature way to handle things. If I'm saying yes, I should say yes and leave it at that. If I'm inclined to say no, I should say no. If it's something to address, it should be addressed later with words, not non-verbal that can cause fights.
On that note, if I'm really really being honest, I think sometimes I used those non-verbals to start fights as a way to be able to voice it.
I grew up with a mom who has control issues. I've been married to my uBPDw for 19 years. I've realized how both of these situations have affected me, including resulting in dysfunctional mechanisms of my own in response. When you can not address things safely in a healthy or direct way, it's a tendency to develop manipulations and unhealthy ways to counter their dysfunctions.
Something we can do is watch ourselves and work on ways we feed back into the patterns of how things play out. We can't change them, but can change how we respond.