How did my pet childhood rabbit die?

It could have died from shock of being held upside-down. Rabbits seem to have a built-in alarm system that makes them die easily of fright. Years ago, my dog was chasing a rabbit. It suddenly stopped as my dog got closer, and it fell down and died. My dog never touched it. It's like a defense mechanism built into the brain to make the rabbit die, just before it is attacked, or hurt in any way, so it doesn't go through any pain and agony. I had a rabbit when I was young, and it got it's foot partially caught in the floor grate, and just died on the spot.