I've seen it before a few times, The rings start fluttering due to high rpm and piston ring groove wear, ( because of the constant high rpm) bit of oil gets in combustion chamber, usually only had a single hiccup, detonation, etc.MY THEORY AND IT MAY HOLD WATER
for whater reason a small chunk of piston alloy,that fractured loose fom the area outer to the dcirclip groove
maybe the wrist pin reached a back and for hammering that stress cracked off a small chunk
of course up and down goes the piston and at the change of direction the recip,the minor bit of metal gets wedged and rolled up to go the other direction
after couple hundred 100,000 recip motions the balled up metal is eroding iron liner material
for sure there was never anything close to melting temps
Piston crown gets hammered, ring grove closes on top ring cracking it in pieces Rings break up and start cutting cylinder , various bits of cast iron get into pin bore.
what red-line are you using? You need to lighten pistons if your going over 12K.
It would be a real good idea to get crank fully checked. Oh, check crankcase bearing bores, you will see fret marks but need to make sure bearing bores are still round. Any signs of piston touching head (pretty common if you have 'correct' clearance) If you don't have any marks on piston crowns or head squish band you probably have to much clearance and con-rod thinks it's a sine wave
Just went back and read all the posts. Pretty sure Cyorg was right about ring groove wear, possibly ring butting (not enough end gap) and resonance. Ring groove wear is something almost always overlooked on 'budget' re-builds where pistons are not being changed as they 'look OK' I've been told I'm OCD about measuring things but really I just don't like 'wasting' good stuff (change it cos your in there) or having engine blow up.