My cat runs away from the vacuum cleaner whether it is off or on. Why is that?
Because they are scaredy cats
My boy cat's not afraid...he just sleeps through it..
Some do, and some don't. Several of mine take off, and others simply watch. It most likely is a noise that is not common, and some cats have a hair trigger spook factor. The slightest change, and off they go. And if they see you moving the vacuum cleaner (even if off), they are smart enough to know where the noise comes from. For people who show cats in shows, they use a vacuum cleaner to determine if their cat will do all right being in a cat show. If the cat shows no concerns, then the hustle and bustle of a cat show will also not affect them.
Your cat isn't simply afraid of the vacuum cleaner. If she had never heard it in action she'd have never developed her fear of it. It is the noise. The noise is probably very unpleasant to a cat plus it may sound to her like some horrible roaring creature. Cats get used to other noisy appliances in homes but there is a difference. The ones they get used to like coffee grinders, garbage disposals, blow dryers etc are used frequently so the cat gets used to it. And the appliance making the noise in those instances isn't a big thing that moves around. Most people only vacuum once a week plus it is a big thing that moves around. So the cat doesn't hear it often enough to lose its fear of the sound let alone get used to the added scariness of it seeming like a big animal. Btw, some cats are not bothered by the vacuum. Amongst other things, I clean other peoples' homes and have met a few cats with a ho-hum attitude about vacuum cleaners. My own three are not exactly terrified but they do stay on the safe side by scurrying off to another room when I turn the vacuum on.
Because vacuum cleaners arent what u think the are. They're really big scary monsters trying to take over the world!!! I think your cat knows something most humans don't. DESTROY THAT EVIL VACUUM CLEANER BEFORE IT EATS U AND UR FAMILY UP... IM WARNING U!!!