Steel shot loads are usually loaded to the max. High pressure. High velocity. That’s what does it. They have to be loaded that way to drive the payload at speeds sufficient for steel to be lethal. Any payload leaving a muzzle at 1350 to 1700 fps is gonna be straight up stout.
I never said specifically anything about the actual shot having anything to do with pressure or recoil. It doesn’t. When I mention “steel loads”, I’m basically saying high pressure, heavy payload, high velocity. All things that’ll loosen up a gun and stress a stock with a quickness.