There are a bunch of videos going around of cats getting spooked by cucumbers, for example here. (For more examples, see this dedicated subreddit.)
Is this merely careful selection from one extreme of the wide range of cat behavior, or do cats have some special aversion to cucumbers?
If the latter, does anybody know where it comes from?
Cats in general are often very jumpy when faced with an unknown or unexpected object. My cat has jumped and hissed at me when I've done nothing in particular except surprise her by being there when she didn't expect it. It's become an Internet meme that cats are afraid of cucumbers, and as a result, people are going out of their way to have cucumbers suddenly "appear" at their pets. I suspect the reaction would be similar to other suddenly appearing objects the cat isn't really familiar with.
A cucumber probably looks like a snake's head/upper body at first glance. If you look at videos and cats and snakes fighting, the cat often uses the same type of high leaps to stay out of striking range of the snake.
I think it really depends on how the cucumber is being introduced to the cat. A lot of the posted scenarios have the cucumber placed behind the cat when it's feeding which can be very unexpected/startling to him or her (especially if they're not familiar with the object).
If you were changing your bed sheets and when you lifted your blanket up, you saw a cucumber on your mattress, I think you'd be startled too.
I agree with @katepaulk that this is just caution around am unfamiliar object. Websearch "kitten apple jaws video" for an absolutely beautiful example of this.
This was apparently a popular question:
Some scientists chimed in to answer, basically speculating it's based on their reflex related to fear of snakes, and/or some cat-specific personality attribute.
