Turned ads back on again to have a look, and it seems when the page first appears it leaves a large (ie deep) space for an advert and then, if a less deep ad appears in the slot, the whole page moves up to fill the gap. If there's an ad deep enough to fill the whole space, it doesn't scroll.
If you run an ad-blocker, it seems to decide every 'non-ad' is a deep one, so every page loads and then moves up when no ad appears.
Very wierd, don't think I've ever seen this on other sites.