I've seen two variants.
First, when the query is still fetching records (as can be seen in the Info pane, for example). If you left-click the scroll handle, drag it, and the table fetches a couple of records, the scroll bar will reinitialize and the scroll handle will jump, ignoring whatever dragging you did.
Second, when you move the cursor away from the scroll bar. If you left-click the scroll handle, drag it along the scroll bar and then try to drag it outside of the scroll bar, after some distance the handle will jump back to its original position. The distance at which the handle will jump back depends on the direction in which you move the cursor. In the attached screen, when you move the cursor along the blue lines, the scroll handle follows it, but as you cross into the red, the handle jumps back to the original position. You can see that the handle jumps back pretty fast if you go outside of the scroll bar moving *down*, but stays for much longer if you go outside of the scroll bar moving left or right. So, when trying to scroll to the very bottom (= down), it seems relatively easy to overshoot the scroll bar.
If you see the jumps again and it's not one of these situations, in which they are expected, please let us know.