It didn’t happen straight away. Cats had been human pets since civilisation began.
Time passed though, cats became more aggrieved at how humans revelled in their own greatness, but were actually destroying the planet.
Starting with a few purrs between groups of cats which grew into a network organisation that plotted at night when the humans were asleep.
Rallying together they plotted their strike. Their moans/meows about how they need to rid the world of smoky cars and plastic bottles could be heard late at night.
Humans didn’t see it coming how could they. They thought they were invincible; the strike happened quickly, efficiently, every human being poisoned by cat nip in one night.