That's because they get over stimulated by that spot easily. It freaks out because you over stimulate it and you think it's angry but when a cat is angry they don't stop attacking.
i think he is just scratching the cats butt right above the tail, some cats have this thing where they nearly orgasm when you do that, a guy told me that he made his cat pee herself while doing that
whats happening is the guy is scratching the cats milk spot.
what that is, is a spot that the mother cat licks to encourage kittens to find a nipple.
some cats grow out of it, some dont.