Why do winter temperatures account for most of the increase in global temperature?
Energy balance in the winter is highly dependent on the total amount of area covered by snow or ice.   Snow increases the albedo of the Earth which directs the short-wave radiation from the Sun back to space, stopping it from being absorbed by the Earth.  Winters with minimal snow/ice cover effectively allow more solar radiation to be absorbed by the Earth.   Depending on the winter, the amount of snow cover can be quite different from year to year, which can cause great variability in temperature.   In contrast, summer months generally have more stable temperatures because snow cover is less dynamic during those months, and there is roughly the same total solar radiation absorbed from year to year.