Depends on the general causation, it its viral in nature (ie. the ZSG), then it would make perfect sense that a government would be the cause of a mass outbreak. Remember that both the US and Russia still have large stockpiles of SmallPox, Anthrax, and so forth and so on... It would most likely come from a less stable part of the world (or part of the world with a possibility for instability) Example, Iran or India...both of which may or may not have bioweapon capabilities.
Color me an idealist, but I have a hard time believing that the US gov't would have any intentional involvement with ZDay
my best reasoning would be (get in a cold war mindset for my explanation)
how better to destroy and/or sew confusion against an enemy population and military without contaminating the environment or destroying natural resources. While the enemy state is busy with the zombie threat they cannot retaliate.
You have to get into that mindset of that era... same era when both the US and USSR had enough nuclear, chemical, and biological weaponry to destroy the world several times over. With this the z-virus (whatever that may be) the enemy would be occupied and defeated indefinitely
I think the weather, I'm originally from Montana and the weather is so messed up all the time there. I think perhaps the severe weather changes could make people sick and start the new plague, the doctors give out antibiotics like candy so we arent going to be able to get better and thus the plague/Zombie apocalypse begins...
I've always liked the idea of a virus as the cause for reanimation. However, I believe the virus would have to be either man-made, or come from space.
One theory of how the SARS virus spread so quickly (before they figured out the real reason) was that it hitchhiked here on a meteor, which burned up in the atmosphere, and spread the disease across the globe. I like to think this would be the origin of the virus, as it accounts for it being widespread and so unique.
Also, check out this article from cracked.com, titled: 5 scientific reasons a zombie apocalypse could actually happen.