They are NOT shooting at just any random car.
They're shooting at any car that overtakes them. Typically they're supposed to shoot into the ground ahead of the car as a warning, and then put a few into the engine if that doesn't work. If the car still rapidly approaches, they tend to walk the bullets into the driver.
The reason they do this is because for every 1 mistake that kills an innocent but stupid driver/family, there are 10 cases where as soon as the first bullet hits the car, it detonates with a huge explosion.
I read a letter from some guy to his dad about his first shoot while on convoy duty. He'd seen the techniques, but this was the first time he employed them. A car was catching up just like in that video. He fired a few into the ground, and the car floored it towards his vehicle. He instinctively opened up (he was manning a 30 cal if I recall correctly) into the car and it immediately detonated. The explosion was big enough that it probably would have flipped or otherwise destroyed the vehicle he was riding in, had they let it approach any closer.
The moral of the story is pretty much if you're an innocent driving in Iraq, never rapidly overtake, never approach an SUV/vehicle with guys taking shots out of the window, and if you ever ever see shots near your vehicle or hear bullet impacts, stop immediately and run like hell because most of the soldiers doing this kind of shooting really don't want to kill anyone, they just don't want to get blown up themselves.
BTW it's not just military and civilian "contractors" that do this... Any Iraqi leader, legit or not, does the same thing when in convoy from one place to another. That's probably why there is no great Iraqi uproar about this practice... The Iraqis pretty much know that active and aggressive measures are absolutely required for anyone of any importance to get around in Iraq, American or not.