I sometimes flinch when I am near stairs. I can imagine falling, and of course that would be scary. I attribute it to my experience actually falling down stairs before. I fell when I was 2 years old, and a couple of times as an adult when I missed a step (I'm a clutz).

What you might be experiencing is a type of anxiety where you fear the worst. If you tend to assume the worst is going to happen (and some people are more prone to do this than others), you kind of get "on your guard" all the time. And then if you get startled or frightened, you flinch much more easily. Kind of like a hair trigger for being startled.

gg