Well remember the 4th rule of firearms, be aware of your target and what is BEHIND it?
A very similar situation happened to me and some friends a few years back. We were out in some farmers fields where we had permission to be from the land owner. We'd been shooting for a few hours when some redneck and his white trash girlfriend pulled up behind us and uphill. At first we couldn't tell what they were doing until the guy cranks a round off at our targets. The round went over our heads and we yelled for him to stop. He told us we were fine. My friend who was a lawyer gave the guy the what for and he yells, "Fuck off!" and proceeds to work the bolt on his rifle and lines up for another shot. My friend said, "Lets go!" grabbed his original M1 Thompson and jumps in his truck and we tore up the hill to the guy. My friend who was livid at this point jumps out of the truck with his Thompson at port arms and tells the guy to drop the rifle and not to move. The look on that guys and his girlfriends face was priceless. My friend then got on his cell phone and called the police. We smelled booze on his breath as well. After 20 minutes or so a deputy shows up and hears our story and then he talks to the redneck and then runs him through NCIC. Guess what? This guy was a felon with active warrants. He ended up being charged for felon in possesion of a weapon, DUI, and then had to deal with his warrants. The deputy confiscated the rifle, towed his truck, and told Ms. Trailer Park to start walking then took the red neck to the county jail. What a great way to end a day of shooting. I'd say in all you were quite lucky you didn't get hit. Too bad more shooters don't take a better look before they shoot.