I tried to reason out the cause and can't say for sure. Last year, I did have heavier then normal mite loads which started earlier, do to the non-existant winter. I had to treat w/formic 2X's.
I migrate between PA and SC so queens don't get much of a break.
I had higher then expected mite loads already this spring so I did a knock down treatment in mid-Jan(sounds early but the weather was warmer then average by at least a couple weeks at that point) before making nucs in March.
Of course, poor nutrition or poor mating conditions seem to play a big part too.
Please post if you find the cause for your losses!