It's a knockdown. The punch landed, and Mayweather's glove touched the canvas.
Maybe there was slight foot contact, but not enough to dismiss the KD. Floyd stumbled because he was leaning in with his feet wide apart. If you get hit or pushed from the side while you're in that position then it's very easy to fall.
The same sort of thing happened against Hatton. Mayweather leaned in while throwing a shot, and Hatton hit him with a right hand counter that sent Mayweather off balance. You can see it in the first part of this gif (not the latter bit, where he falls backwards):