It could be that they think it is a snake. It would be a weird snake though. What would be a natural predator for a cat?
There reaction is what you would expect if they saw something they fear. Maybe they just don't like cucumbers. They have a strong smell.
Cats seem rather nervy and "wired", much more than dogs are.
They are always a little bit on the lookout for dangers. Something about them recognises they are possible prey. Most dogs don't feel like that.
I wonder if anyone has tried the cucumber experiment with dogs? If I still had one (a dog I mean), I would try it....if I had a cucumber!
They react very strongly. Dogs are a bit of an apex predator. It is the wolf in them. Wolves hunt, they are not usually the hunted.
Smaller cats can be eaten by snakes and birds of prey.
We were feeding a bearded dragon on Saturday and she suddenly "leaped to safety". She saw the big free roaming iguana at the reptile centre. Iguanas are vegetarian, but bearded dragons are from Australia where there are large predator monitor lizards and birds of prey that could come from above. It was an instinct because this dragon is born in Toronto.