We have good years and bad years with varroa mites
My theory is the most virulent mites killed the colonies they infested. Last
winter saw heavy losses in many parts of the country taking both colony and
their parasites with them.  This leaves mites that are less damaging or
slower to damage to their hosts.  After all a good parasite doesn't kill it's
host if it will die with it.
Coincidence and causality are two different things but in the long run there
is no advantage to varroa in killing off all of the honeybees.
Cun grano salis

Tom