That's a pretty rude grab to get you to stop. Neither productive nor respectful You say you don't have a fucking clue what the problem is - but you know it has something to do with making coffee for this coworker... or that's the trigger for your partner. Something must be so bad if she felt like she had to break up.
Is she afraid to talk about how insecure she feels about your slight past with this coworker? I am guessing is another woman? Is there anything else to your relationship or do you pretty much only make coffee? Like, is there anything that would be a breach of boundaries to your relationship? Which may not be defined, but maybe she's still reacting poorly to/not regulating emotions that arise from whatever. You have to talk to her to find out.
But the trick and problem is part of the communication dynamic here - I mean, I don't know everything between you two. How do you ask questions, if you do; what are her reactions to those questions and how does she handle it all? And where does this exchange of behaviors and ideas get you two? What are common denominator issues in that dynamic? If you tackle that, you might find a knot where miscommunication is occurring. And maybe this is part of a wider problem for her and that makes the subject all the more sensitive.
I think claiming you don't have the balls to stop making coffee is a fucking stupid remark. And frankly, it's fucking sexist bullshit. I might dump someone on the grounds of that command alone, you know? I would absolutely talk to them about it - I don't want to be with someone who's going to bully their way to 'resolution,' which is manipulative behavior. I've been manipulative in my past through verbal and emotional abuse - it's not always an explicit intention and can just be a destructive habit that developed as a defense mechanism, but that doesn't diminish how serious of an issue it is. If she can't be sensible, empathetic, and chill the fuck out to talk about what the real problem is - and be sure you are taking part in this too! - but if these things are not exercised with communication, it makes it difficult to communicate at all. If this is how she acts at 40, there is something behind all that.