Air Bag question

I was riding behind a new Jetta that was being towed. It had apparently been involved in a minor accident, the right rear quater panel had been hit and just the passenger side curtain air bags front and rear been deployed. I always thought that when the car felt an impact strong enough to deploy the airbags, ALL the bags were inflated. By looking at the Jetta it seems that the bags were only inflated on the "hit" side of the car. How do they really work???