Why Cassie was mad at Big Ma?

Mr. Simmons demands that Cassie apologize to his daughter, although Jeremy insists she already did. Cassie tries to run away and is met by Big Mama. Mr. Simms demands that Cassie say "I'm sorry, Miz Lillian Jean," and Big Mama reluctantly makes her comply. Cassie says it and runs crying into the wagon, thinking this is the cruelest day that she has ever endured.