That knife must have some sort of inhibition enchantment that renders its foe catatonic
Classic conditioning. Give the cat a bigger weapon to think about so that it can use it to slit your throat while you sleep.
That was unexpected for the cat too
The cat can stop them whenever they want. But they won't, cause of the implication.
Oddly enough, this works the other way round as well - if your knife isn't letting you pet it, very deliberately place a cat next to it and it'll instantly behave!
What do they do?