why did the last play bruno and gretel performed with grandmother end in disaster?

Bruno remembers the last play they had put on, about a week after Father's promotion to Commandant, had ended in a dramatic argument. It had been Christmas Day, and Father had decided to wear his new uniform.

After dinner and the production, Grandmother, the only one who disapproved of Father's new uniform, had said to her son, "I wonder if all the performances I made you give as a boy led you to this. Dressing up like a puppet on a string" (90). Grandfather had encouraged her to be quiet, but she had not obeyed. Mother had tried to calm her down by asking her to confirm that Father looks handsome in his new uniform, but Grandmother was incredulous at the suggestion that how he looks would be important. Mother had told Gretel and Bruno to go upstairs, but they eavesdropped from the top of the stairs. When Father called himself a patriot, Grandmother had yelled, "A patriot indeed! The people you have to dinner in this house. Why, it makes me sick. And to see you in that uniform makes me want to tear the eyes from my head!" (93). She had stormed out of their house, and Bruno hadn't seen her since.