We turned to go, but then Miz Thomas said, "That Mitchell, he done somethin'? He in trouble?" "We just want to talk to him, Edna," Hammond assured her. Still, though, as we rode away, I saw Miz Thomas frown, and young as I was, I knew she was worried. She was worried because my brothers had come. My brothers had come asking about Mitchell, and my brothers were white.

—The Land,
Mildred D. Taylor

Based on the novel’s historical context, what causes Miz Thomas to frown?