I know everyone is different etc but any clues as to why this is?
It's a way to regain control. I do not mean this is any demeaning way towards your ex. But she's most likely gaining control again. If you stand your ground, she will try all sorts of tactics; blaming, aggression, helplessness, crying after you, becoming stoic, etc. She's not going to change without years and years and years of therapy (depending on how bad her BPD is).