That high proportion of very damaged mites seems rather unusual.  The  
background in unselected colonies is about 20% of fallen mites showing  
some damage, and most are less damaged than that.  If they are light  
colored (translucent to yellow) they are young or immature mites, some  
possibly removed by hygienic workers out of brood.  If they are mostly  
reddish brown, that suggests some kind of active removal/grooming of  
adult mites off adult workers.

Looking at mite fall with some magnification can drive one crazy.   
There are numerous possible combinations of various states: age  
(ranging from light to dark brown), condition (dried to "plump"),  
damage (ranging from a nick on an appendage to just the carpace), even  
live vs. dead if the board is looked at right after removal from a  
colony.  Interpretation of what gives rise to these different stages  
seems fairly speculative.  Some have settled on just counting the  
older mites and categorizing their damage to measure grooming.  Other  
recent papers use proportions of various conditions (such as ratios of  
young mites/old mites) to gauge resistance via hygienic removal of  
brood mites.

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