As the mouse pointer moves among the menus and submenus under the Start Menu in the Explorer shell, submenus automatically cascade after a set amount of time the mouse arrow rests at a location with a solid-black right-pointing arrow.

This time is set in milliseconds in the MenuShowDelay line of the Registry. The path is

HKEY_CURRENT_USER\\Control Panel\\desktop\\MenuShowDelay

Somehow this value was set to over 65,000 which led me to believe that something was wrong with my mouse!