Here is one of the possible reasons http://talk.newagtalk.com/forums/thread-view.asp?tid=463988&mid=3771980#M3771980

 As you can tell from the charts snow cover kind of insulates the ground surface from the air temperature. What helped with the snow cover was the standing spring wheat stubble. One more thing that might have played a role was soil moisture, after wheat harvest in mid August there was about 8 inches of rain before freeze up. They say that wetter soil don`t freeze as deep compared to dryer soil.