When my colonies collapsed it was at the end of a very productive season.
Productive for mites as well as honey.  I did not like to treat while honey
was on the hives.  My theory at the time was that because the developing
larvae in the late summer were parasitised they were not able to lay down the
fat bodies etc to turn them into "winter bees",  thus considerably shortening
their life span.  I did not notice any obvious signs of virus such as
crawlers.
Chris Slade