I don't think this is a BPD thing, actually. The thing about BPD is that it magnifies any underlying emotions. So, if he has this quirk where he twitches, it is more than likely just something he does when he is under stress. Since he has BPD, you probably see it more often than you would in someone who doesn't get upset so easily.
My wife pushes her lips really hard from one side to the other when she is feeling stressed out. Another thing she does is make a clicking sound with her mouth, and, finally, there is the fast finger tapping... . If I see or hear any of that from my wife, I make myself scarce if at all possible. Who knows how many episodes aimed at me I have avoided due to this observation? I shared my observation with her once... . I thought for a minute to myself, "now, why would I tell her this? She might stop doing it, and I find it kind of handy to know!"
I think that even if she tried, she wouldn't be able to stop herself. Habits like that aren't done on a conscious level. So, I know how to better understand her as a result. It's funny how these things can actually be empowering. BPD trait or not, get to know your SO and what they need. It can only help you and your r/s!
