I think some of it stems from the idea of hierarchy in the family.
My MIL was very much into the idea of being The Matriarch. Her mother had been, and she saw it as her rightful place.
Her son refused to dedicate his life to her church, or become a pastor. But once she had a DIL, she knew that would fix things. Daughters HAD to listen to their mothers!
Her son didn’t get married until his mid thirties, and then married ME, a single parent who was used to running her own life. Neither of us had any interest in following her orders. I even refused to side with her against my husband! And continued having babies after she told me not to! (No, she had zero to do with our finances). I wouldn’t even call her Mom, and had the AUDACITY to tell her no. Frequently.
To her, it was very much like having put in decades toward a promotion, and right when everything is in place, the position is eliminated without warning. Pretty much all of her behaviour was centred around attention seeking and control. Things she believed she was owed, especially because she adopted my DH. (Not speculation. She constantly told him that growing up, and repeated it to me). She felt cheated, betrayed, because her expectations and demands were denied.