Try flicking to about the middle of the page normally. Then flick up and down like youre looking for something. Not slow but not too fast. Still happening to me even though i closed the page and opened it later.
I've figured it out (I think). The scrolling action accelerates the more you scroll while the page is moving. So for a long page, the first flick pushes down 1x, the next 1x, but by the fourth or fifth flick (might have to scroll up if you're reaching the bottom of the page, from what I saw it still continues to accelerate, even after switching direction), the page is zipping up/down. Good as a replacement for a scroll bar, I guess, for sites like GameFAQs.
It might also depend on where you catch the scroll, I.e. how far down/how far you wait before flicking again.