Mike,
Some interesting things happened there.

My gut feeling on this is that the gun had not closed properly and that the bolting was only just engaging by a whisker. By this I mean that the top lever had not come back to its closed position and was sitting way off to the right. This would result in the bolting not being correctly engaged.
Maybe one of the two loaded rounds prevented the action from closing normally because it was tight in the chamber.
This could be caused by insufficient resizing of the case body, shoulder too far forward, neck wall too thick etc.
If the gun has not closed properly, and the bolting is not fully home in the barrel bites, there will most likely be a gap between the barrel and action faces which will result in excess headspace and an off face condition.
This could have caused the seperation when the gun was fired, even if the pressures of the handloads were within spec.
It's possible the shooter may have bumped the top lever slightly as the gun was fired, or the bolting may have released from the jarr of firing the gun if it was only just engaged by a tiny amount.

This is why it is very important to ensure all cartridges correctly fit BOTH chambers of double rifles.
This is done with the barrels removed and cartridges are checked in both chambers to ensure that they do not protrude past the barrel face and that they freely drop in and out of the chambers with no resistance.

Hope this helps.
Gotta go get dinner but I'll get back to this one later.