I would think that, at the very least, the Airbags should have deployed.
Warning: The following paragraph describes some of the physics of airbags and may be disturbing to some.
The airbags probably did deploy. Unfortunately, air bags are not designed to replace seat belts; they are to supplement lap and shoulder restraints to help limit head and chest injuries. The air bag initially inflates at speeds of up to 200 miles per hour, and if a passenger or driver is not fully restrained they are likely to contact the air bag before it is fully inflated and may sustain serious or fatal injuries.
We should always remember to:
*Use both lap and shoulder belts on every drive. Always. No exceptions.
*Wear the lap belt under the abdomen and low across the hip. Never wear the lap belt above your hips.
*Keep the lap belt snug.
*Wear the shoulder belt portion down over the collar bone away from the neck and cross over the breast bone. Never position the shoulder belt so that it presses against your neck.
*Move your seat as far back as practical, especially drivers smaller in stature.
*Try to keep a 10 inch margin between the steering wheel and your breast bone. If you cannot keep a 10 inch margin, then try: Tilting the steering wheel downward--this will aim the air bag at your chest instead of your head and neck; Slightly reclining your seat back--this may help you achieve the 10 inch margin. If, however, reclining the seat back hinders your view of the road, raise your seat.
*Keep unsecured objects in the car to a minimum. All unsecured objects have the potential to be deadly missiles in a crash.
Just a thought, but someone could temporarily remove their seatbelt while driving. Say you were too hot and tried to remove your coat while driving (bad idea). The same action of removing the seatbelt to remove a coat could lead to making a steering mistake that causes you to leave the road. Similar if you removed the seatbelt to reach into the back seat to get something. I have no idea what actually happened, but throwing out an idea of what might make more sense here.
I think we all are guilty of doing unsafe driving practices occasionally. Let's remember that the seconds we save by not pulling to the side of the road and parking before **removing a coat, reading or answering a text, checking that we have what we need, restraining our child, or whatever we might do that interferes with the task of driving** is not worth the years of life we might lose.