Me too!I am just guessing. It must have to do with the nervous system that the back collapses first. It could also be that a heart shot reacts like a major heart attack which jerks the animal backwards. But I have also seen many heart-shot animals that take off at full speed just to collapse about 100m later.
The kudu in my avatar was a heart shot and he staggered around for 20 or so seconds before he went down head first. When he was cleaned the trackers held up the lungs for me to see what was left of the heart, nothing but threads of meat were left. If you didn't know that it was his heart you would never know.
I've never seen this before very often.Why in the world, when you hit to your game right in the heart it falls on its hind-quarters first?