Question about "How it Ends"

So what exactly happened that is causing all this in the movie ?

Never even heard of it, but here is a video with spoilers/theories. But just looking at the premise, I doubt if exactly WHAT is causing the disaster is ever revealed or even matters.

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"Film is a disease. When it infects your bloodstream it takes over as the number one hormone. It bosses the enzymes, directs the pineal gland, plays Iago to your psyche. As with heroin, the antidote to Film is more Film." - Frank Capra

So it could have been a volcano eruption that caused this apocalypse but you are right that it doesn't matter. This film is about what happens and how people behave when an apocalypse occurs, basically it turns into every man for himself .