You describe the classic indications of a severe over-pressure rounds and incipient case head separation.
The 7mm Mag is a belted case and should, in theory, be head spacing on the belt, so shoulder position shouldn't affect much. I don't know much about the belted mags but I think I've read that many of them DON'T actually head space on belt. If that were true, shoulder position would matter. Someone else can help you there.
The first part is certainly classic pressure/head separation though.
Unknown rounds of unknown age could be carrying deteriorating powder that will not burn predictably. If they are reloads, the powder could be perfectly normal but a severe overcharge. Either way, don't shoot them anymore and stick to known (your own) reloads or factory ammo.