250-500cc Broken friction plates on my '11 wr 300

I discovered 3 broken friction plates recently while inspecting my clutch. The friction plates are original oem mated to a Rekluse Z-start pro installed last spring with the low rpm engagement/hard engagement rate setup using Pro Honda 80 wt oil. Oil level was correct. It's been a great set up so far. 2200 miles on the bike and oem plates with about 500 miles on the Rekluse. Most recent ride before the discovery was a 2.5 hr harescramble in very wet and muddy conditions with a few long technical hill climbs and some very fast sections on and off the mx track and in the woods. Also lots of 3rd and 4th gear lugging through the muddy and technical sections to minimize wheelspin. I suspect they shattered during the race. I heard a pop and the tranny went into neutral but I just shifted into 2nd and kept going with no further issues. I even rode it down my street after the post race washing and didn't notice any clutch issues. Is this normal with broken friction plates? The oem clutch basket is moderately notched and was a little less so when I installed the Rekluse, but I never experienced any abnormal shifting or clutch behavior.

So what do you suppose caused this? I'm planning on filing down the basket notches. What other measures are needed to prevent this?

Thanks!