I can understand why that would have been disconcerting and frustrating. Here are my thoughts.

1) The underbolts weren't fully engaged to begin with, and the shooter didn't notice it because the gun is new and somewhat stiff, and he's unfamiliar with it.

2) Due to lack of experience with the rifle (and, worse, my guess is probably the type as well), he used a shooting technique wholly inappropriate for the type, which resulted in the thumb of his shooting hand hitting the toplever under recoil causing the action to open.

3) I'm with Tinker on the glove. I think the likelihood of No. 2 above is exacerbated by the use of a glove on the shooting hand. I can't imagine doing so with a double rifle. On the forward hand, yes, but on the shooting hand, no. The larger bore the double, the less sense it makes to me. For this to be the cause of the incident, the shooting hand had to be moving around some on the gun during recoil, and that shouldn't happen.

4) The handloaded ammunition remains an unknown. The fact that the loader has handloaded professionally for 20 years isn't germane at all, unless he's substantially experienced in doing so for double rifles. If he isn't, the ammunition is suspect.

5)

...and he's leaving on a hunt for three of the big five in 12 days? It would have been impossible for him, or the rifle, to have been ready for him to make this hunt with it. Especially if this is his first double rifle (and it sounds like it might be), he's not even gotten started yet. I hope he has an appropriate rifle that he's more experienced with to use on the hunt. This one needs to stay at home.

I don't mean to rain on this gentleman's parade at all, but the description of the incident above makes me think that he's new to the type and has underestimated the learning curve. I watch new DR shooters regularly. The transition is often bumpy.