Cats are afraid of cucumbers because cucumbers look a bit like snakes.
Cats are hardwired through their instinct to ward off snakes so the cat becomes afraid of cucumbers because of the similar look to snakes.
Cucumbers look enough like a snake to have the cat's instinctive fear of snakes kick in.
Whether it's a noise, a stranger or a smell, cats don't like surprises.
They are particularly frightened of them. However, there are many GIFs and YouTube videos which show some surprising surprises.
One of the most common is placing a cucumber next to a cat and watching their exaggerated responses.
A cat sees that cucumber is a threat, and when confronted with a "threatening situation, cats respond with fear and anxiety.
This instinctive reaction is visible when a cat is surprised by a cucumber: jumping up or, conversely, freezing in place; fur standing up; yelping and so on.