I was watching The Walking Dead a couple of weeks ago where Glenn met Abraham Ford's group en route to Washington DC, and there was a part where I almost had to laugh: just before they cut to commercial, Ford introduced one of his party, Eugene Porter (with a apocalyptic mullet), and said, "He knows exactly how this whole thing started." Well, howdy doody, boys and girls! I guess we can all go home! By that point, given how irrevocably fucked the world is, does it really matter how the apocalypse began?
Does it matter how any apocalypse in fiction begins?
Some would say...maybe.
Audiences like to have answers, even if they're really crappy ones like the trees were just warning us. Yeah, I'm looking at you, M. Night. Other times, the story gets by without an explanation at all. Yeah, I'm looking at you, Cormac McCarthy.
In rare cases, such as The Road or the 2011 film The Day, a mystery cause works perfectly fine because within the confines of the story, the cause is unnecessary. The world went to hell. People turned on each other. Survivors must endure the wackiness. Survival is almost always the universal theme of apocalyptic stories.
In other instances, the cause is integral to the story. Even though The Walking Dead doesn't explain how the zombie plague began, what we do need to know is that zombies themselves were the reason for civilization's downfall. What creates them isn't even important anymore, whether they're created from voodoo curses or government viruses...or radioactive contamination from a probe returning from Venus. Yeah, I'm looking at you, George Romero.
In some cases, such as alien invasion stories, the details of the apocalypse become even more important because the threat is a thinking one that plans and forms strategies. In order for the writer to get the most bang for the buck, the capabilities of the aliens need to be known. Do they have force fields or ray guns? Are they looking to annihilate all of mankind, or can a truce be made? Do they even think like we do, or are they a hive mind like the Borg?
So yes, no, maybe so...the cause of the apocalypse is only important if your story demands it is. If it does, hammer that stuff out and stick to those notes. But be warned: overthinking the cause can lead to long bouts of procrastination. This is one of the reasons why I've avoided writing science fiction lately. Too much research can lead to some very cool ideas, but nothing that serves your story.
Think about the story first, and then retcon, reverse engineer, and backronym your disaster.