The first change, simply enough, is that the mousewheel is no longer a button. This was a mistake of history, and we’ve corrected it in SDL 2.0. Look for SDL_MOUSEWHEEL events. We support both vertical and horizontal wheels, and some platforms can treat two-finger scrolling on a trackpad as wheel input, too. You will no longer receive SDL_BUTTONDOWN events for mouse wheels, and buttons 4 and 5 are real mouse buttons now.
From comment in glumpy source