Looks like detonation to me. A few reasons why detonation occurs is Cylinder temp too high,Very lean mixture,Low quality fuel(low octane) and too much spark advance.
The section on the skirt which is blue and carved out looks to have been caused by the flame travel.
I have made a lot of passes with the wifes car, SBC 383 with KB Hyper-U-Junktic pistons using nitrous. Goes 12.4's on motor, 11.5's with a 125hp shot. Wrong fuel, 93 Super Unleaded, wrong plugs, FR2LS AC (extendeded tip), ran 12.0 A/F ratio on wideband and only pulled 4 degrees. Can't believe I never melted that thing!
From my own experience with hyper pistons... looks like you lifted a ring land. Maybe not on this pass but some time recent. From there the flame and pressure were gonna go down. Remember something about the NX kits. The horsepower ratings they give with their tuneups is rated for wheel HP. So look at that 150 shot as really close to 200.
Lifted a ring land, probably on an earlier run. This causes a bad ring seal so oil gets in past the rings, detonation occurs and burns the piston up next pass. You were probably too rich to begin with.