Which of my cats are weird?

I have two cats (both female), and one of them is doing a very odd thing.
One of my cats stands up when I touch it just behind the tail (on its back, so it normally stands up whenever I stroke it). It's very odd, but it always purs whenever I do it (Even when it's sleeping and I wake it up by tapping there- it immediately stands up and purrs). It's obviously not voluntary (sometimes it's trying to sit, and it stands up, then tries to sit, then stands up, as I'm stroke it). My other cat, though, has no reaction whatsoever when I touch it there.
I'd always presumed that it was a natural thing for cats, I'd never really noticed that one of my cats does it and one doesn't. So which is the weird one? Is this normal behaviour for cats or not?

Welcome to PT! :)

I think what you are referring to, is what we are calling "Elevator butt" :D It's quite normal, most cats do it, and I'm sure they do it because they're telling us how they enjoy to get stroked there.

Please tell us more about your cats, we love stories and pictures, if possible. :)

Neither of your cats are "weird"! Do they have sexes so that you can refer to them as he and she or even better, names!? Where in England do you live?

LOL! Elevator butt! That's a good one! My little girl,Belle does that. Thor, not so much...but now that I'm talking about it...if we make a blowing noise or blow on Thor, he'll start flipping out like he's on catnip. He pulls himself along the carpet and if you're not careful, he'll swing at you...which he would never do normally...he never has hurt me...has anyone heard of this before??? He'll also start chewing on anything in front of him..usually the corner of the coffee table :)
Why is he doing this? We were thinking that maybe it sounds like hissing...I don't know...stumped. My husband enjoys doing this to him..it's quite comical, but I don't like him doing it too much :)