This sometimes happens with starting loads in the flanged nitro expresses also. It's due to low pressure, not high. The .577/.450 headspaces on the rim like the nitro expresses do, so I doubt headspace has anything to do with it. I think the difference between modern boxer primed cases vs the original Berdan primed with it's much larger cap plays a role in this as well.

"Serious rifles have two barrels, everything else just burns gunpowder."