One speculation is sudden massive pole-shift of the Earth on it's rotational axis.(physical shift, not magnetic). Think of the Earth as a spinning, precessing top in space. For whatever reason, perhaps a near-miss by a passing asteroid, e.g., it's rotational axis is altered and regions formerly temperate are suddenly shifted much farther "North" in a matter of only a few hours.
Such a cataclysmic movement would probably also result in massive wind storms (hundred's of MPH) as the Earth's atmosphere "slides around" to catch up with the land movement. For the new northern regions these winds might also contribute to rapid, "freeze-dry" type cooling.
Lot's of stuff on the net and in books about cataclysmic earth changes, e.g. Velikovsky, and others.
Ahh, yes.
Alan Eckert. The HAB Theory
I have it and have read it a few times, but not recently. At one point in the book, scientists worldwide table a conference to discuss the pros and cons of the theory, and the corroborating evidence.
Those chapters contain a great deal of scientific fact - much of it still unexplained.