It looks like you just had an open pile of gravel sitting on the trailer which is apparently illegal. I did something similar this weekend only got in trouble. I brought home 2.6 tons of 18" landscape boulders on a utility trailer. Since I had old 2x10's around, I added them to form sides on the trailer, so none would roll off.
On the way home I pulled over to let cars get around me, and one motorist pulled over and started screaming at me that I was dropping sand and gravel on the road and it was damaging her car, and she was calling the police because I should have had the load covered. Although it hadn't caused any damage, she was bound and determined to make me pay for it.
The problem was that when the loader dumped the rocks on the trailer it put a small amount of quarry floor sand on the trailer as well, and this is what was falling off when I hit bumps. An hour later ( and a $75 ticket later /forums/images/graemlins/frown.gif /forums/images/graemlins/frown.gif) I got home.
Then to top it off, one of the 420 loader lift cylinders blew its
seal while I was unloading.
Although not intentional, I can see that I was scattering rocks, and had I been the one getting hit, I wouldn't have been happy either.
Next time I think I'll just let the quarry deliver it and save the
hassle, risk, and headache.