When you look at the trailer it has what was called a "suicide door", hinged on the aft side.. When it came open the wind over-powered her and pulled her out.
I suspect, since they were en-route to somewhere to watch the eclipse, there were more people than seats in the tow vehicle.
This is a very tragic accident the driver will never forget. In Indiana the driver is responsible for the safety of all passengers.
For instance if the vehicle is pulled over and a passenger is not wearing a seat-belt, the driver is ticketed. Likewise with people riding in a truck-bed, driver is ticketed.
As to Airstream design; when you look at their website, some are hinged on the forward side, some aft. It depends on floorplan and awning arm positioning. The Airstream involved has an awning arm beside the door so close if the door had hinges on the forward side the door would hit the awning arm.
I do suspect a design change in the Airstream future.