Illness, death, and antisocial behavior. All of these were threats we evolved to handle, people who are "a little bit off" in one way or another, who might endanger the group or individual. This, and that our pattern seeking brains don't like it when something doesn't easily fit within an existing schema, even more so if it lies just outside of our existing preconceptions.
Obviously, I can't say that these definitely are the reasons why we experience the uncanny valley, but I think it's probably a better explanation than... Skin walkers? Or whatever else the meme would be implying.
Still, it's a cool premise for a horror story at least.