Offhand I would have thought being 75 feet away was enough. He was unlucky because a good size piece of shrapnel headed right towards his leg. Maybe part of the lawnmower blade? He lost one leg below the knee.
I am surprised they could not reattach it. Medical science has become quite good at that nowadays.
I feel for the guy. He was just incredibly unlucky. With 360 degrees that thing could have gone, it was an amazing stroke of bad luck that it picked that tiny little spot where he was standing.