It was a whitetail shot in the South. The question should have been:
"Does anyone have thoughts about what caused the animal to suddenly drop?"
I can only assume the initial shock from the bullet to the vascular system causing a sudden drop in blood pressure (something that does not always happen in a heart or chest shot). The carotids were shredded, most heart shots I've seen aren't completely destructive, rather cause a slightly delayed response from the body.
I'm also more accustomed to watching them run for a ways before stopping. I am just surprised a bit because it's unusual from my experiences and the hunters I know.
Sorry, no pics. stopped that a long time ago, but I did keep the antlers for the kids to play with. I'm just sorry neither one of the boys were with me that day.