I'm guessing you're onto something Brent, as it pertains to the angle of entry. I figured it wasn't simply bullet knockdown power lifting the buck up and over on his back. More likely that he simply reacted to being hit at that angle and died as he was on his hind legs. But it was pretty impressive, regardless.
My niece's husband was with me and was hunting with his Savage 110 in .243 caliber. He looked at his gun, and said, "Maybe I should rethink this modern bolt action?"
His reaction at seeing the buck flip backwards shouldn't be printed here. But what he said was hilarious.