According to About.com, the car that has hit the debris in the road is the one at fault. Insurance companies typically consider object-in-the-road accidents to be avoidable accidents.
About.com explains that in the instance in which something flies out of one car, and the object hits a second car, the first car is at fault if the object hits the second car before hitting the ground. If the object hits the ground first, the second car will be held at fault.
According to FindLaw, the police are likely to pull over a driver whose trunk is poorly loaded, or who is driving an open-bed truck with items in the bed not safely secured. The driver of such a vehicle is likely to receive a mechanical violation ticket.