Which of my cats are weird?

[Oppolo]

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?