The weather systems of the planet are highly complex.
This is the reason it's so hard to predict weather -- which is short-term and local -- but easier to predict climate, which is long-term and regional.
It's like the surface of a pot of boiling water. You can't predict whether a bubble will or won't appear at a precise tiny point on the surface exactly five minutes after you turn on the heat, but you can predict what the overall surface will look like overall -- it will be bubbling.
However, modern meteorologists are getting better all the time, and among the partial explanations they now give of the immediate ("proximate") causes of the cold snap are a) that the "rivers of air" -- like the jet stream that flows east to west in the northern hemisphere -- have developed some big dips to the south, which pull down Arctic air into lower latitudes, and b) that something called the AO -- short for Arctic Oscillation -- is relatively weak at the moment.
Since the AO's winds tend to swirl around the north pole and, when they are stronger, keep the cold Arctic air penned in up there, when they are weaker, it can let more frigid air spill down toward the south.
Scientists often advise that, when thinking about weather, or climate -- or anything, really -- it's always important to remember that nothing ever happens for any one reason.