1. The AI Apocolypse
We cannot have a list like this and miss out on the biggest theory of all, artificial intelligence will one day take over the world, right? Well, not exactly. An Apocolypse the likes of science fiction movies is highly unlikely, especially with advanced weaponry like EMPs capable of stopping any robot or machine for bursts of time. On top of this, humans are the ones who make AI.
If anything, AI will always be a victim to what made wants it to be. Even if it develops intelligence beyond what it is equipped with, this does not mean it will decide to kill all humans. Unless someone makes an Ultron-like being, it simply will be impossible to see. We will not likely see a being the likes of Ultron because making something like that would effectively be committing suicide.