I'm making a lot of assumptions here & that's not normally ok with me, but I'm going to throw out what my spidey-senses are saying anyway.
Is it possible that she's the one pushing this - insisting on being there in your face in order to calm her own insecurities? That when he told her about the classes & your insistence on driving DS there, she melted down on him & demanded to be included somehow? If it was his idea, wouldn't he have defended himself to your mother a bit more instead of just "ok" & walking away? Her bad mood = angry at her lack of control over this situation?
Again - assumptions here, not facts.
