Dead things, and behaviors that don't align with our predictive models shift the context to one of threat - if something shaped like something you understand starts behaving in a way that you no longer understand, you'll become progressively more concerned. If a pencil started rolling around aggressively chasing you, it'd evoke fear, even though you'd probably defend yourself fairly capably.

If enough predictive models are broken, people feel like they've gone crazy - various drugs and experiments demonstrate a lot of these factors.

The interesting thing about uncanny valley is that the stimuli are on a threshold, and humans are really good at picking up tiny violations of those expectations, which translates to unease or fear.