It happens to me when I start at the last post of a thread and then try to scroll up. A few seconds after scrolling up, it jumps back down to the last post. It doesn't seem like it matters how long after the page loads I start scrolling. Then I scroll back up again, and when going from the last page to the second to last page (or any page n to n-1), there's another jump - this time jumping seemingly randomly further upthread.