Interesting article in Psychology Today about zombies and why they fall in the uncanny valley realm (though he doesn't use that term).
Pattern Recognition. That guy is staggering, so perhaps he is drunk. But wait! That kid is also staggering, and kids don't get drunk. And that woman is staggering; when was the last time I saw three staggerers at the same time? Things are not fitting into my usual patterns. I do not recognize this pattern, and I am therefore forced to switch off automatic and to perilously fly manually. Most of the time we're flying by instrument, but not now. Now, we need to look around.
...
And fear sprouts from the depths of your brain, your primitive cortex freaking the hell out and your frontal cortex madly searching the hippocampus for anything even remotely familiar.
And this is where you experience horror.
Horror is the initially familiar becoming increasingly unfamiliar. And the easier it is to question what you're seeing, the less familiar it is by definition, and the more familiar it was at first.
You can read the whole thing here.
