What causes Stacey to call Cassie "cracked"?

Answer: Stacey doesn't understand why Cassie wants to talk to Lillian Jean.

Explanation:

Stacey knows that Cassie is still hurting from Lillian Jean's insults, so when Cassie sees Lillian Jean on the walk to school and follows her to talk, and even pretends to believe black people are actually inferior to white people, he doesn't understand the reason behind it. He doesn't know that Cassie has a revenge plan.