It's in quotes because that's the one word I can think of... it doesn't sound quite as final as "the apocalypse," but I'm having trouble coming up with the right word.

Anyway! What do you think caused this catastrophe? Do you think it was related to nuclear weapons, or extreme weather catastrophes? Maybe something environmental?

I know they implied what happened in the preview to the movie; but of course, that's not necessarily what happened.

It could have easily been like in the dark ages. I have read a man's theory where he believes that our world is going full circle, whatever happened in the past will happen in the future (still trying to understand) and it might be a plague. Like in the dark ages. Never know. Could be a war that happened. Everybody keeps saying they have the weapons of mass distruction.
I'm not really sure, just spitballing.

The movie version strongly suggests the effects of “nuclear winter”. Near the beginning, the man looks out to the horizon to see a series of bright flashes (missile strikes?). He immediately fills all available containers with water. After this, the family seems to be staying under cover as much as possible, using only their own stored resources. These last two suggest the fear of radioactive fallout.

Multiple nuclear hits would throw enormous volumes of dirt and dust into the air, blocking the sun for some time, lowering temperatures, and leading to what some have described as a nuclear winter. This could last for some time, and may end up killing most plant and animal life on the planet. This is also suggested in the movie by a continual black, dirty looking, brooding sky, and low temperatures.