The same thing that causes it in summer - thunderstorms. According to the thunderstorm map, there have been quite a few of them lately.
Sometimes, storms that have moved hundreds of miles out into the Atlantic can still play havoc.
https://thunderstorm.vaisala.com/tux/jsp/explorer/explorer.jspSome years, November through early December is the best period of the radio season. I can recall numerous years when band condx were good, but a weather front moved through the week between Xmas and New Years day, bringing with it stormy weather, that never quite went away for the remainder of the season. There was one year back in the early 80's when the entire winter was more typical of summertime condx than winter.
We can only hope that the present high static period will go away, and leave us some more quiet band condx during January and February. But I wouldn't bet life or fortune on it. March seems to be a toss-up. Sometimes it is quiet and winter like, but other years it is noisy and spring like.
I check the thunderstorm map before going out to the shack, turning on the heat and firing up the rig. If the map shows a high amount of storm actvity within 600-800 miles, I usually don't bother.