> It may be the sky-high phoretic mite infestation rates in colonies just prior to collapse that allow such substantial invasion.

Took the words out of my mouth. Hives which are collapsing from mites are bleeding off mites, which are instinctively leaving the hive because the host is dying. Additionally, bees may be abandoning the hives as well and joining healthy colonies. There is ample evidence  that viruses cause bees to become disoriented. 

In fact, there are examples of parasites causing abnormal behavior, so it is plausible that sick bees are unwittingly migrating to healthy ones, and healthy ones are unwittingly picking up mites from weak hives, causing a rapid buildup of mites in healthy colonies, despite having low levels of mites only weeks earlier. 

PLB

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