i will agree with Gill here and say it is because she doesnt like it. Most cats have somewhere that they dont want you to touch for any length of time. If you stroke a cat from head to tail and see the back ripple it is because it irritates and generally the cat will allow this to happen until it just darn right gets on their nerves! lol
My older cat will only allow certain areas (mainly chest & chin upwards) to be stroked at length, my younger cat will happily allow you to stroke his belly (as well as head area) but has "ripple back" - not sure about the tale but it could be connected with the stress the cat feels when people touch or continuely touch her hind quarters. Whereas my lad would think nothing of simply ripping your hand to shreds to relieve his stress at my invading behaviour
Worth a try and the tail might not get as much attention??
I only know this after several hand rippings and i have to monitor mines temperament quickly and change areas or stop altogether. It works. He's happy - i get a few less strokes than i want but after 10years my hand is still a hand and not a bloodied worn down stump