When uncle hammer asks Cassie about her visit to strawberry Big MA does everything she can to stop Cassie from telling her story Why?

Big Ma knows that Uncle Hammer can be impulsive and quick to anger, so she wants to protect Cassie from any potential conflict or trouble that might arise from her sharing the story of what happened in Strawberry with him. She likely believes that it is best to keep the situation under control and avoid any further confrontation.