Are there more accidents in winter months, and if so, why?
I'd argue fall and spring bring more unpredictable weather, and slippery runways are year-round in the tropics.

That being said, for all jet (commercial) accidents, the weather-related contributing threats are:


Meteorology (e.g., failure to identify threats before a flight) 30%


Windy conditions 16%
Poor visibility 10%
Thunderstorms 9%
Icing 1%

Poor braking action (e.g., slippery runways) 10% (falls under airport facilities, not meteorology)


All figures are from PDF page 172 of the IATA 2017 Safety Report.

Given the opening statement, [near ground] icing is more attributable to cold winters than the rest. The rest are not limited to Dec–Mar in the Northern Hemisphere. The report does not attribute the seasons to the accidents.