"did not find any evidence of mites"

When I see or hear statements like that, my immediate response is, "That settles it, the cause was mites"
**All hives have mites **, and failure to detect evidence  means categorically that the evidence was *missed*.

Randy Oliver has very precise instructions on how to turn a brood comb and inspect the upper cell walls for the mite feces deposits of bright white Guanine.    http://scientificbeekeeping.com/first-year-care-for-your-nuc/

I keep bees in the Big Sur and Morro Bay area, just south of your location.   The fall of 2015 was a hideous season for bee parasites, with very high numbers and many collapsing hives.  The mild 2014-15 winter (no freezing chill) was likely a contributing factor (mites had an early growth advantage).  Climate stress was real.  However, the coup-de-grace on my deadouts was  virus and parasite decline.  My mortality from disease came in the 2nd-4th week of November.  

I treat most of my yards, but run two yards without treatments to test the various fantastical and fraudulent claims of the hobby gurus.   The  treatment-free yards suffered severely this year. One was freshly  populated on new equipment by VSH F1 and F2 daughters from AI queens, which I had hoped would express resistance to mite diseases -- hives in that yard, quite unexpectedly expressed Tracheal mites (verified with dissection).   Tracheal mites can depopulate a hive at the fall turnover very, very quickly.   The other untreated yard is populated (mostly) by the mystical wild feral bees taken from remote Big Sur traps.   DWV expressed in those hives, but I am hopeful that some may shake off that viral infection.

My county (just south of yours) has significant number of AHB (African) hybrids, especially in the interior.   These bees tend to "migrate" in the fall -- the hives depart and overpower others, temporarily usurping the attacked colony.   The prime "migration" and usurption period is September and not November.   The migration and usurption behavior is likely to expand into domestic colonies as more and more hobbyists trap problematic feral swarms.   

Robbing was severe this fall (drought related), and a colony that robs out a mite-weakened neighbor can spike in mite population very rapidly.  The increasing density of poorly managed hobby hives on the heels of the sudden "hipness" of hobby beekeeping, means these sources of infection are rapidly expanding.

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