I think part of it was Walt inadvertantly got his off hand (not the one he tried to block the shot with) in the little pipsqueak's face and extended that hand/arm a bit as the guy was going to the floor - kind of like pushing off whatever object was in the way to help keep his (Walt's) balance. I'm not sure what explanation the 3 stooges gave, but there was clearly no malicious intent, just incidental contact to the head/face area.