Baseball and softball size clusters in March (dead or alive) have been
common in New Hampshire for at least 10 years.  Here is an explanation of
how it can happen.

If varroa populations are high in September, the long-lived winter bees
which are being raised at that time will be harmed and have their life
expectancy shortened.  When these bees are approaching end of life, they
have the urge to leave the hive which they will do even at 30 deg F (-1 deg
C) in February.  If the beekeeper treated for varroa in late September or
early October, a post-mortem in March will reveal few if any varroa.

Cold weather inspections/disturbances and certain types of winter feeding
can add stress and thereby reduce lifespan further, so sometimes it takes a
combination of varroa in September and beekeeper meddling in February to
produce a dink in March.  Been there, done that.

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