Some theories via:
 via bee-health.extension.org

One aspect of resistance is grooming behavior. Moretto et al (1991)
found grooming behavior of African bees in Brazil to be eight times
more efficient at removing Varroa than Italian bees and 31 percent of
infested African honey bee workers removed Varroa by their own or
another bee’s grooming action (Moretto 1997). These results were
confirmed by Arechavaleta-Velasco and Guzman-Novoa (2001) in Mexico,
however, Vandame et al (2002) only found eleven percent of mites
removed by African bees compared to eight percent by European bees in
Mexico. Therefore alternative attributes are possibly conferring
resistance. Workers of Africanized bee colonies are more efficient at
removing Varroa infested brood than European workers in the same
conditions, displaying superior hygienic behavior (Moretto et al 1991;
Spivak and Gilliam 1998; Vandame et al. 2000 and 2002). Shorter
development time of pupating African bees (10-13 hours less than
European bees) is also suggested to limit the reproduction of mites
(Camazine 1986; Ritter and De Jong 1984). All of these factors are
expected to reduce the reproductive potential of female Varroa mites.

Swarming quite certainly isn´t the reason because the mite population
stabilize. That means: Mites have different survival/reproductive
rates in AHB." However, mite populations in similar-sized AHB colonies
stabilize at 1000–3000 mites per colony [3,5], allowing colonies to
survive indefinitely (L.M. Medina, PhD Thesis, University of
Sheffield, 2003), although the resistance mechanism, until now, has
remained elusive"

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