Because there isn't any such an event which will do that, the content loaded in the WebBrowser control is based on your HTML content, the HTML document that you have loaded into it. So the mouse interactions are controlled by the HTML document, rather than your Web Browser and forcing the control to take actions in such cases is not a good idea.
To do that, I will suggest that you edit the web page that you are loading. JavaScript provides functions and features that you can use to load the content when user reaches a position on the document using the scrollbar. For example, this one
http://jscroll.com/[
^].
You can also handle the mousemove events and determine whether mouse is at the bottom most location in the HTML DOM, do not track these changes using the WebBrowser control, it is of no use!