Cats are naturally terrified by anything that sneaks up on them without making any sound. Cucumber appears dangerous to cats. Cats associate cucumbers with snakes which are deadly predators that can attack them.
Account to Jill Goldman, a certified animal behaviorist, cucumbers seem like a snake to a cat because of their green color and elongated shape. Since the cat did not see the cucumber approch or make any sound, the cat is likely to leap in fear.