> What was the adaptive behavior/selective pressure involved in varroa's jump from apis cerana to mellifera? A target of opportunity or some genetic change?

The host switch required no change, western honey bees were similar enough that mites were attracted them and they provided a naive host with even better living quarters than the Asian honey bees. In August of 1999 the following appeared:

Host–parasite relationships 

The association between Varroa and Apis cerana appears to have been a long one, and the mite rarely causes harm. On its natural host, mite reproduction is almost exclusively restricted to male (drone) brood. Male brood is frequently absent from colonies and is restricted in numbers even when it is present. 

Thus, during those periods when males are absent, the mites must exist phoretically on adult workers, and are unable to reproduce because of the lack of male brood. Further, if multiple mites invade a drone cell, emergence rates of the host are reduced to 28%. Any mites that do parasitize worker cells are likely to be detected and eliminated, and workers groom phoretic adult mites from each other. Hence, mite populations are held at a low level in A. cerana.  

Conversely, effects on A. mellifera colonies are usually catastrophic. Although mites have a strong (on average eightfold) preference for parasitizing drone brood, they successfully reproduce in worker cells, especially when drone brood is absent or heavily parasitized. Reproduction on worker brood leads to an exponential increase in mite numbers. 

Varroa is poorly adapted to its new host, A. mellifera. Commonly, mother mites fail to oviposit on pupae of A. mellifera. Predictions about the future impacts of Varroa are bound to be wrong, so I will refrain from making any.

Oldroyd, Benjamin P. "Coevolution while you wait: Varroa jacobsoni, a new parasite of western honeybees." Trends in Ecology & Evolution 14.8 (1999): 312-315.

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