by the looks
most of the time when I've fixed arctic cat motors with this look its the result of DETONATION and its been going on for some time, maybe 50-100 miles, maybe since new.
the most likely cause is lean fuel air mixture causing the toooo lean fuel mixture along the outer edge of the piston to explode with a bang instead of the ideal soft push ( technical term as pointed out detonation/deto ), most likely an air leak. Seals on these engines don't go bad sitting for that length of time. Air leak from base gasket very likely, loose stud bolts/nuts. Leaks around the reed valves/reed cages, blown a few belts, crank seals with belt threads stuck in them.
Just a little detonation now and then for 900 miles will anneal the piston crown which allows the ring lands to widen and the rings to flutter and not transfer heat off the piston crown, then under a load in the rpm range where the deto is the worst , a ring end may weld itself to the piston wall for a mili second, break off and go around through the transfer ports and dingle up the outside edge of the piston.
However I would expect when you pull the piston, you will find rings intact, skirt intact. When the center of the head shows little or no scaring, the outside edge of the piston is eroded from little bits of aluminum becoming liquid for a moment and bounding around . The pop you heard was the last straw, not the start of this problem.
Timing could cause this, but the timing on these motors is same for both pistons, one good piston one bad piston, rule out timing.