House cats average 10 to 12 pounds, they're on the small side of the feline family, so while they are predators, they're also prey. So they've got basically the same startle response as a squirrel: Explode. Leap into the air limbs akimbo and scramble around until you land then figure it out from there. Aeons of natural selection has apparently selected that as the most successful strategy for small surprised quadrupeds.