You are on a Mac machine with a non-Mac mouse.
There is an issue in the way that browsers on OSX handle non-Apple mice. The problem happens across different browsers. The problem is that for some reason non-Apple mice provide different information to the browser about how far the wheel has been turned. With Apple mice (or with any mouse on Windows) the amount of scroll that is reported is the same for the main panel as it is for the parking lot.
Because it's not possible to tell the difference between someone scrolling very slowly with an Apple mouse, and a non-Apple mouse that is scrolling as fast as it can, it's not possible to solve with heuristics. To date we have not been able to find a solution that does not cause problems for folks with other browser/mice combinations.