Me and my ex uBPD had a phone argument, and she literally turned into a different person towards me overnight. I literally could do no right, couldn't talk to her right, couldn't look at her right, all of a sudden she had a long list of things that I did wrong. I'll never forget the dead look in her eyes and the dead silence as I tried to reason with her, as if any type of logic could not get through to her in her current state of emotional dysfunction. I literally thought i was speaking to a different person, did an alien take over her body?
I went through something identical. Our last phone call was around an hour long, in which she just listed everything she thought was wrong with me, and any response I gave (which ranged from talking engaging, saying "I understand how you feel", and then reduced to "OK") was wrong, and only seemed to make her angrier. She then hung up and blocked me, mid-sentence.
This wasn't an argument, but a rant. I haven't heard from her again. We went to uni together, and she just avoided me completely - although I noticed that she often looked very withdrawn and sad in class, almost remorseful/ashamed of her behaviour and treatment of me (or maybe that's what I wanted it to be). Either way, I haven't heard from her, and she has me blocked everywhere.
From reading this thread, it does seem that this is a highly common pattern of behaviour at the end of relationships with pwBPD - I personally just put it down to shame/guilt, as someone above mentioned. Doesn't make it any less hurtful, though.