I believe that the author, Cormac McCarthy, deliberately left the catastrophe ambiguous. To have a known cause to of why the world was in such disaster would change the message and meaning of the story. It's about a man's reaction to destruction, not the events leading up to it. If the author would have given us a direct answer to what it was caused by, then the book would be completely different, and would've focused more on the physical reaction compared to the emotional one.
At the point that I am at in the book, nothing has led me to believe that only one thing caused the catastrophe. I believe that it was a mix of war and natural disaster working together over time to magnify the destruction. The ash and fire being constantly discussed
