it is unfortunate, but for most it is a yearly event. you may never know why. could have been robbing that you missed. queen could have been lost late. heavy mite load. combination of all.... starvation is common, but if you knew they were ok going into winter, it's a little early for that.
try not to feel bad. i have already lost two. they were two that i knew where iffy, but it's still a little sad.