One of the latest trends on the Internet is for people to sneak up behind their cat and place a cucumber behind them. Then when the cat turns around and sees it, they often freak out, jump in the air, or run away in sheer panic. The theory is that cucumbers resemble snakes and cats are afraid of snakes.
However, the real answer might be the sheer surprise of suddenly seeing something close behind them that wasn’t there before. Imagine walking into your kitchen and turning around to see someone standing behind you who wasn’t there a moment ago. So it’s less likely that cucumbers are the reason cats jump but because they’re simply surprised.
Just don’t scare your cat for your own amusement because cats have a way of exacting revenge, and you never want to get on the bad side of a cat.