The ads pop up right underneath your finger in the lag between deciding to scroll and your finger touching the screen to drag. With the page jerking around underneath you that seems to end up being classified as a click rather than a drag, and away you go to whatever piece of crap you really don't care about.
Finger down -> finger up = touch
Finger down -> move -> finger up = scroll
How does a scroll get interpreted as a touch? It sounds like a bug to me.