This is not at all a high pressure load. Both the Ramshot data and QuickLOAD put it about 26 Kpsi when all the data is properly tweaked for matching barrel length and velocity. Enforcer is actually a slow powder for the application, and burns very inefficiently in it. That is part of the problem, since spherical propellants often have ignition issues in low pressure loads. The WFN bullet form is stubby, so there is about 25% empty space in the case, which exacerbates slow ignition.
The outer edges of the primer are not mushroomed, but they have sharp edges at the corners where the piercing is located. They correspond to the chamfer in the firing pin bushing, which could be expected to cause primer cratering in a high pressure load. Further, I notice the whole primer looks somewhat concave or inverted cone shaped around the pin indentation, another low pressure sign.
Further confirmation of the low pressure problem comes from the fact the velocity prediction from QuickLOAD matches the published pressure and velocity data for a 8.275" barrel, but shows 200 fps greater velocity with a 4" barrel achieving the same peak pressure with 20 grains of Enforcer than Totaldla measured. That tells me he is not getting up to the pressures Ramshot or QuickLOAD get. Indeed, QuickLOAD shows a 20 grain charge would only be getting to about 17,000 psi if it produces the velocity Totaldla measured. This is clearly an ignition issue.
So, why did a low pressure load pierce a primer? When a gun is fired, the primer gas cannot instantly squeeze out through the flash hole, so the primer, acting like a little piston, pushes itself back out of the primer pocket. Chamber pressure later reseats it by backing the case up over it. In this case it looks like it flowed over that fat firing pin creating the inverted conical profile whose base happened to correspond to that chamfer in the firing pin bushing which offered not support to the edge of the primer. My speculation is that simply created too sharp a bend in the primer cup, encouraging it to crack.
I think I would not choose to use Enforcer with that bullet weight with lubricated lead bullets as they are unseated too easily and therefore offer less start pressure than jacketed bullets. I would also not use thin primer cups in a gun with that primer-diameter firing pin bushing chamfer.