1) Gestures. If you touch the upper edge of the screen, iOS will interpret this as a "jump to top". It's very easy to accidently trigger and has been the bane of my existence when I was still actively reading on my mini.
2) During loading, Safari will back and forth between loading images. This settles once the images are loaded, however, some ad providers keep ads on a rotation. Safari will keep jumping to the latest new ad-image. As far as I know it's only Safari that does this (Apple likes to be very 'yes-please' towards Ads). You can try one of the free browsers instead. Maybe Dolphin? Been some while and I'm not up to do date on which ones are decent.