I have read a number of different accounts of winter kills this year. Many of these accounts said the hives were pretty clean and free of bees / mites on the bottom boards.
The clusters were smaller then expected, since the fall checks showed good numbers. Bees were clustered, but pulled away from food.
My thoughts are that there maybe two factors at play here.
#1) This summer / early fall was very dry in many locations. The lack or reduction of nectar may have reduced the queens egg laying rate or stopped it all together for a time.
During the fall inspections there would have been a number of field workers toward the end of their life span and would have died later the fall. These bees would have been cleaned out when possible.
So, the actual population of young bees for winter was smaller then desired.

#2) This winter came early and hard in most locations. With temperatures that have been consistantly cold.

The combination of lower population and colder weather has caused the cold starvation of many of these hives.

It's just a thought. Any ideas or comments?