How and why is Cassie acting strangely in the beggining of chapter 4?

The children do not want Mama to find out that they were resposible for the school bus of the white children breaking down. One Sunday, Cassie helps make butter and hears Mama and Big Ma talk about how she and the children have been acting strangely for several weeks. She cannot tell her mother what is wrong because she and her little brothers promised Stacey that they would say nothing about the bus. When she breaks a dish, Mama sends her to find the boys.