First off we need more info on this happening to decide the cause definitively! However I see several clues that nobody has mentioned yet. First, the only way the loaded side can be EJECTED, is if that barrel was fired! The shooter's thumb being on top near the top lever is another clue that would only open the action if the action was not fully closed, and it seems from the discription, the left barrel was the one fired. This would exasorbate the movement of the top lever to the right, because of the barrel riseing up, and to the leftpushing the lever closer to the thumb. The action opening during early combustion, could posibly allow the chamber pressure to blow the case, because of the preliminary opening of the action. With this happening, the fireing pin on the unfired chamber being slightly out of line with the primer, failed to fire that barrel too, luckily, but relaxed the tumbler, releasing the ejector. This could be why the loaded round was ejected, with out actually being fired.

The owner of this rifle seems to be in the dark where double rifle use is concerned, and if the pull is too long, then the stock needs to be shortened, and someone needs to show him how to hold a double rifle. The other thing I'd look at is the ammo! As 400 said, there are many well known handloaders who know absolutely nothing about loading for a double rifle. Nobody has said what bullets were loaded in this ammo, but I'd almost bet it was monolithic solids, and if so, this could be another contributeing factor, with the rifle not well closed, and the excess pressure developed by the monolithic solids could have contributed to this gang bang!

In any event, someone with some double rifle smarts needs to look this rifle, and ammo over!