Read the excerpt from Roll of Thunder, Hear My Cry. "Now, there ain't no sense in going around being mad. You clear your head so you can think sensibly. Then I want you to think real hard on whether or not Lillian Jean's worth taking a stand about, but keep in mind that Lillian Jean probably won't be the last white person to treat you this way." He turned toward me so that he looked me full in the face, and the seriousness of his eyes startled me. He held my chin up with the wide flat of his hard hand. "This here's an important decision, Cassie, very important—I want you to understand that—but I think you can handle it. Now, you listen to me, and you listen good. This thing, if you make the wrong decision and Charlie Simms gets involved, then I get involved and there'll be trouble.”

Based on the text, what causes Cassie’s father to have a serious talk with her?