The bullet had to hit something before hitting the deer causing it to break up. It would not have to be anything big to cause that. I have seen that before. The sabot reaching the deer is very unusual and I have no way of explaining that as I have never seen that. Even a tight fit sabot to the bullet isn't going to stay on there at 1700 FPS +. How far away was the deer when you shot it?
jlane, bore butter in a barrel shooting sabots is not recommended, even by the makers of the stuff. The friction between the sabot and barrel is what provides the accuracy with the rifling. Smooth or loose fits are not your friend in a MZ.