OMG! I can't belive what I just read.
I guess Duke just saw something running in the grass and thought it was a wild animal or something? I know when the rabbits and cats play outside the cats acts different with the rabbits (by pouncing on them and biting them) and the dogs used to do it with the cats and rabbits (they just ignore them now).
So maybe it's just something about being outside that makes them think differently of them, or maybe he was just "playing" with him.
I'm so sorry you had to witness this too.
R.I.P Loki