In about January each winter, a number of hive entrances become completely clogged with hundreds of dead bees.
And often at the same time, bees are flying even though the weather is freezing or 'just' above freezing- sometimes on dark cloudy days. Not normal cleansing flights. Usually one bee at a time.

Generally these hives will die before spring.
They have sufficient weight, plenty of honey leftover in spring to start a new hive off.

Any guesses as to what disease I should be researching?
The bee inspector looked at one dead hive last year and called it winter kill, however I'd like to find out exactly what is winter killing them. Thanks!