If the tissues on the ends of varroa legs ("tarsal pads" or "ambulacrum", or whatever) is physically degraded by OA, this would be easy to see.

I think more likely that the pads (because varroa "Secrete a Fluid over Their Adhesive Pads", per the paper I recently mentioned), are one of the very few places where the otherwise well-armored mite can be affected by the chemical.  It may well be that this is also how Formic Acid works (rather than the claimed degradation of the waxy coating on the mite's exoskeleton and the eventual dehydration of the mite due to loss of integrity of the coating, yet something else that one might expect the mite to evince under physical post-mortem).

...on the other hand it was about a decade before the research community noticed what Bob Harrison pointed out early, often, and loudly - that his varroa could not be Varroa Jacobsonii, because their bodies were not circular, but instead, oblong.

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