If something falls off your vehicle and strikes another vehicle, you are responsible. Unlike kicking a rock up from the pavement, which is considered road hazard, for which you have no control. I payed for many a vehicles damage, do to my insured losing items from their vehicles. From beds, to a wheel, (axle broke off and went into oncoming lane), and even a chicken coop. Don't ask it wasn't pretty. If the item bounced up and hit the vehicle, it will be a comp loss, but you can bet your last dollar that the A party's insurance WILL subrogate against the B party, to get their money back.