pwBPDs are ultra-sensitive and extremely inflexible, which means that anything they don't like at that particular moment can set off a huge reaction. You can be doing something in exactly the same way as you've always done it, but their reaction may be completely different. Unfortunately we can't change any of it unless they feel something's wrong with the way they express themselves.
This is so true. I deal with the same thing when it comes to my kids (his stepkids). You are not alone. I have stopped trying to tell him he shouldn't feel the way he feels. There's just no point. One day he will freak out about my son drumming with his hands on the table, a week later it won't phase him at all. It's no longer about trying to figure him out - now it's just about protecting my boundaries when it happens.
It is like he walks around with a magnifying glass and super hearing.
OMG I can relate to that... . he is highly sensitive to sounds and seems to notice everything.