Final answer:
In 'A Good Man is Hard to Find,' the car accident is caused by the grandmother's sudden movement, which distracts the driver and results in the car overturning.
Step-by-step explanation:
The cause of the car accident in the story 'A Good Man is Hard to Find' by Flannery O'Connor is distracted driving. While the family is traveling, the grandmother suddenly remembers an old plantation house that she had visited in her youth. Excited by this memory, she distracts her son Bailey, who is driving, by jumping up and startling him, leading to the car overturning. This event is pivotal as it leads to the encounter with the Misfit and the ultimate demise of the family.