Although I still think the contrast & place-tracking stuff is part of it, I'm realizing the want-to-fiddle thing is a big factor for me too. One thing is that when I use a trackpad (makes it sort of cumbersome to select text), I find myself idly playing with the two-finger scroll, pushing the page up and down even though I'm reading something in the middle of the screen. I think it's the same impulse.