Cold air is more dense, than hot air. In winter the difference between the room air temperature and outdoor air temperature is much bigger, so the density difference is much bigger, making Archimedes force stronger and the airflow stronger as well.
BTW, that's also how stove pipes work - vertical column of air hotter than surrounding accelerates well and creates a strong airflow feeding the fuel. But if the pipe is thermally conductive, that air will cool down and won't flow, for hydrocarbon fuels it can also condensate water, that could extinguish fire underneath it. So, pipes are made insulated enough for strong flow.