[Craig T]

Originally Posted by gacook

I think it's less to do with them being afraid of cucumbers, as being more "shocked" that something which wasn't by their food bowl (safe place) when they walked up, is now there.

Originally Posted by GH85Carrera

I wonder if you put any object down behind them that was not there before they walked up to eat if they would have the same reaction.

Cat owner, experiment please. Try random objects please.

I'm sure this is it. A cat is obviously extremely aware of it's surrounding, like all catch-and-kill predators. They must let their guard down when they're eating. It's likely just the surprise. Bet it would happen with a crescent wrench or hair brush just the same.