what horrible thought does the grandmother have that causes the car accident?

The Grandmother remembers an old plantation that she thinks used to be in this area. Bailey does not want to take a detour to go find it, so The Grandmother makes up a lie about how there are secret doors in the house with hidden treasure; this makes June Star and John Wesley scream and complain until their father agrees turn around and drive down the dirt driveway. However, after they have been driving for a while, the Grandmother realizes that the old plantation is actually nowhere around there at all. Her reaction causes the cat to escape from its box and jump on Bailey's shoulder, and he veers off the road.