We don’t know why he rammed them off the road. As said above, the road is dangerous and people speed and drive crazy on it. Could maybe have been the shooter was angry because the family was driving the speed limit and were in his way.
It is likely that there is no valid or reasonable explanation. Many, if not a majority of people these days have no consideration for anyone, particularly when they are driving. Our society has been in decline for my entire life. I do not expect the trajectory to change... especially with people from rougher parts of the world making up a larger and larger percentage of our population.