As stated earlier a lot of my gaming revolves around apocalyptic events. In fact, many of my hobbies revolve around the end of the world. From reading, to writing, to gaming, to watching movies I have considered the nature of such an event and below is one of the conclusions I wish to share.
The cause of an apocalypse can be broken down into three broad categories. From these broad classifications, the nature and consequences of an end of the world scenario can be extrapolated and defined.
While these classifications may seem obvious once stated, I have read far too many books and games about the apocalypse where the author seemed to have no idea why the apocalypse occurred. This leads to inconsistencies in their work and these inconsistencies prevent the suspension of disbelief. Even if the author does not fully disclose the nature of their Armageddon to the reader they must fully disclose it to themselves.
Natural: This covers natural diseases, meteors randomly striking the earth, super volcanoes exploding, solar activity, etc… No entity has a hand in causing the apocalypse.
Unnatural: Manmade and alien disasters fall into this category. Tailor made viruses, nuclear war, nano-technology run amok, planet wide invasions, etc… Entities, not of deity status caused the apocalypse.
Supernatural: Acts of God, Ragnarok, The Great Cthuthlu rising up from the depths, any destruction for which a deity type entity is responsible.
For example an asteroid breaking orbit and crashing into the planet could fall into any of the three classifications depending on the reason it fell. If it bumped another asteroid with no outside intervention and fell out of orbit then that would be Natural. If an alien space ship used a tractor beam to pull it out that would be Unnatural and if the hand of God plucked it and sent it hurtling to Earth to smite the wicked that would be Supernatural.
Once again the exact cause does not need to be known by the reader or even the survivors of the story, but the author must know “the why of it” to present a consistent story.