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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.

Other hominids as well as visibly diseased humans.

Corpses. Bodies not dressed as deceased imply existence of a threat.