When you have such a small dog experience a traumatic situation like that where the body goes into shock, it could be something as simple as the brain shutting down due to overload and therefore the body will shut down. To something as complicated as the spinal cord behind the skull (there is a small opening...a soft spot if you will) being damaged by something he may have hit when he rolled after tripping. The point is we all will never know and it is unfortunate and I'm sorry for your loss. Please try your best to not dwell on exactly "what" happened, you know the "how" and despite the fact that it occured over a simple trip, I wouldn't let that slow down the next little guy that comes into your life.
I really feel for you as I lost one of my little guys way too early in life. He had just turned 6 months and had a freak accident himself, I know for a fact he'd still be here today if I had a small portable oxygen tank at home w/ me and that hurts to this day.
There is a saying on my little ones tombstone: "THOSE WE HAVE HELD IN OUR ARMS FOR A LITTLE WHILE, WE HOLD IN OUR HEARTS FOREVER."
