How does papa's leg get injured?

At first Stacey will not say what happened but finally tells the other children that on the way back from Vicksburg, the back wheels fell off the wagon, probably a trick played by some boys he saw standing near it in town. Rather than unload everything, Stacey held the horse still while Mr. Morrison held up the back of the wagon and Papa attached the wheel. A truck with its lights off approached them from behind, and they didn't hear it because of the storm. It suddenly turned on its lights, Papa grabbed his shotgun, and someone in the truck shot Papa, grazing his temple. The horse reared up and the wagon rolled over Papa's leg.