Answer: It seems like unusual behavior so I wonder if they were exposed to chemicals (cleaning the flagstone?) or excess nitrogen? Since Irish moss is easy to divide, take a few small slices from your healthy plants and plant them where the others died. If they die, too, there's something in the soil that's preventing good growth; if they live, the ones that died either weren't able to root well or something was spilled on them.
Best wishes with your moss!
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