I have a theory but a lot of people wont like it. When a pwBPD starts being nice again then they are probably getting supply from somewhere else.
I think what he means by this is that at this point they are already involved with a possible replacement.
Under my circumstances i wouldnt say she was getting nicer, but i just noticed a change in how she acted around the time she started courting her friend's husband as a potential replacement. The rages and everything were still there but something just seemed off to me. In retrospect that was it.