I'm bored so I figured I'd put up the most likely chance as to how humanity would die off.

I personally think that it would be along the lines that the interaction of magnetic fields causes our magnetic field to destabilize and flip, as has happened at irregular intervals throughout the planet's history. Leading to two major things for Humans:

- We'd Lose quite a bit of the upper atmosphere, especially in the ozone layer. The Magnetic field interruption would allow ionizing radiation to destroy atmospheric molecules and carry away ionized particals. On a much less significant level, you should also keep in mind, Earth's mass is finite. Any mass that is lost will not be replaced, except by meteor/comet impacts of course. Anyway, Back to the main topic, depletion of the upper atmosphere would mean much more severe radiation reaching the surface of the Earth, and all of us being touched by it. This lost mass would have to be replaced via natural processes and could take decades or longer.

- Tectonic upheaval could occur easily. Our planet's rocky composition is largely thanks to the fact that we exist in the "metal-rich" layer of our galaxy, and the "metal-rich" layer of our solar system. The motion of our rocky surface plates is directly related to the alignment of our planet's magnetic field. The reversal of the magnetic field would cause worldwide quake and volcanic events. Possibly even the big one, and I'm not referring to anything resembling that earthquake movie or the events it depicted. This could throw tons of methane, sulfur dioxide and carbon dioxide into the air, blocking out the sun and causing an ice age.... Which would mean we would be screwed.