I think there are many different reasons, the bigger picture of course is climate change. More concretely, air is getting warmer at the North Pole (due to glaci

I think there are many different reasons, the bigger picture of course is climate change. More concretely, air is getting warmer at the North Pole (due to glacier/ice melt, open water reflecting less sun etc.), which reduces temperature gradients (those that fueled the Yet Stream that was responsible for the stable Polar Vortex). Grant scale of ocean water temperature change is another reason, and (more specifically) more chance for more snow in Siberia. The north pole weather system is connected to all other weather systems, so it is not easy to define a clear causal chain.