He was to the side of the vehicle, it clearly says he opened the door, commanded them to stop, however it was in drive and she attempted to flee. He then fired.
Correct. He may have had his center of gravity towards the door. As the vehicle began to move forward, he fell forward. Hate to tell you, but most cars have a set of back tires on a rear axle. Had he fell down and his leg (or more) positioned under the car, he could be killed. Therefore, he is in fear of his life and has every right to discharge his weapon.