The replacement of low-fecundity Japanese race Varroa with a high-fecundity Korean race in Brazil demonstrates the selection pressure for greater virulence in Varroa.

The selection of high-mortality DWV strains over low-mortality DWV in Hawaii, northern Scotland, and South Island New Zealand demonstrates the selection pressure for high virulence viral strains through Varroa transmission. (these three processes reported in separate  papers).

Varroa are nearly clonal (as the usual mating is older brother to younger sister).  This encourages selection for dispersal through horizontal transmission because a single resident foundress in a low mortality-low virulence bee colony would not colonize and would not compete well for selection.

Varroa may have selected for a single dispersal event, the fall collapse period.  Bee robbing behavior peaks at this time, a mite that manages its parasitism to coincide with this event has a selection advantage in the race to colonize new hives, the hives with the highest likelihood to survive the winter isolation period.   A "non-violent" mite that goes into the winter in a weak, but not dead, hive may perish with its gentle host 50% of the time -- that strips away the kinder, gentler mites in favor of the mean killers that jumped ship in October.

Ecologically, I think a good case can be made for a selection event in Vladivostok for extreme virulence.   A. cerana and A. mellifera were placed in niche competition.  A. cerana could avoid mortality through swarming and drone preference, but if it hosted high mortality Varroa it had greater fitness in its niche.  This is a "Typhoid Mary" strategy of relatively greater survival in an environment with pathogens.  

If the original inter-species competition selected for virulence in the Korean race, there is often "selection inertia" that results in over-emphasis of the pattern.  The wild hive bees and domesticated apiaries may be in a similar competition, resulting in continuing selection for high-mortality in the least resistant member of the dyad.

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