In the book Darl is considered by many people to be strange and maybe even crazy but he is somewhat smart and can figure out peoples secrets which would be considered eccentric behavior in their family because he is different. He seems to disagree about taking his mothers body to Jefferson unlike the rest of the family. This alienates him and makes others think he could be crazy and when he decides to burn the barn down with his mothers body inside it this shows the reader how he might be crazy. All these things can justify his family in thinking he is crazy and committing him to an insane asylum due to them thinking he is crazy and different.
This helps the book be more provocative because to the reader he may not seem super crazy but for his family it does seem crazy. This allows for more thought provoking conversations about the book as a whole and its themes and messages.