My partner with BPD said she was sorry some stuff did not get put away.
I said I brought to much over, you got overwhelmed that is why it has been there for weeks
Now my partner is angry
These are explanations that come to mind:
Is it possible that she didn't feel validated by your answer? Maybe she didn't feel overwhelmed but something else and is angry that you assumed that she felt that way? Or she felt like you brushed off her apology by taking some of the blame yourself?
Maybe it could also help to take a more solution orientated approach to those conversations. Instead of 'explaining' why things are as they are, offer encouragement or a possible solution instead? (e.g. "I appreciate your apology and I know it is a lot, maybe you can do it later/tomorrow/some other time?") My bf sometimes genuinely doesn't know how to schedule choires he needs to do and gets frustrated. It helps if I propose a plan that we can discuss.
Not sure if this makes sense and applies to your situation, for the moment I agree that it's probably best to wait it out.