I'm having a hard time believing that anyone could accidently bump a top lever from the fully closed position to the open position during recoil, even with an inexperienced (to doubles) shooter, with a gloved trigger hand, with a .470, or whatever else you can think of.
No matter how bad the hand position or the technique used, it would be damn hard to have your thumb push that top lever all the way so the gun flys open during recoil, if the lever was in the fully closed position to start with.
That's a sideways movement which would not be easy to do with the gun recoiling pretty well straight back.

It would be a very different story if the gun was not properly closed, and the top lever was sitting in the almost open position.
In that position the top lever is sitting out to the right.
If the shooter had his thumb lying on the top strap, and resting against the top lever, it would be quite possible for the recoil to drive the the top lever against the thumb and cause the lever to open the gun.

My feeling is that the bolting was barely engaged when the gun was fired and the shooters thumb was incorrectly positioned on top of the gun and resting against the top lever.
This would also explain why the fired case seperated.

It's easy to "blame" the shooter due to his possible limited experience in handling double rifles.
Keep in mind that we were all new to doubles at some time.