but as you can see there is no room for tumbling or falling back
Will have to disagree on that one. If the people against the front bulkhead were not secured to the aircraft, then on a rotate such as was observed, they could well have ended up in the laps of the people at the back.
None of the parachutists were restrained in any way, they were sitting on a smooth floor. To consider that they would not move aft when the floor suddenly goes to at least a 45 degree slope aft is pushing the bounds of probability.
They would not all be against the back bulkhead but the C of G would move far enough aft to give the scenario that was observed.
What is your theory as to why this crash occurred? Why did it happen if everything had gone as hundreds of other jumps that had been carried out using the same equipment had had no problem?