After I read the book, and the catastrophe was left ambiguous, I was afraid the movie might consider trying to provide an explanation for it (glad they didn’t). But I thought if they really felt they had to, my own pet theory that the Yellowstone Supervolcano erupted and shrouded the planet in winter for years would’ve probably been the best.
To me it feels so much more in line with the gritty verisimilitude of the story and less like a blockbuster/scifi cliche such as a meteor impact or a nuclear winter (of which the later would bring with it radioactive fallout, which didn’t seem to be a concern), especially considering the sheer amount of fine, gray ash covering everything.
Also, speaking of the movie, during the flashbacks with his wife, wasn’t there some muted, distant rumbling continually droning in the background?