Playing Columbo here:
1) If it was connected as you say wouldn't the drawbar, PTO shaft, and implement actually prent the tractor from "flipping" over backwards as that is where the buddle of equipment is?
2) Wouldn't the drawbar be the first thing to hit the ground like a "wheelie bar" on a dragster and stop the flip?
3) With the drab bar connected how can the tractor flip backards with a hardened steel shaft preventing the backward motion like a wall brace when building a house?
I could go on but I'm boring myself!
Mixing what we know with some speculation:
According to the story, the PTO shaft was chained to tractor and a tree. There was no mention of an implement that could stop a backwards flip.
Being chained, the chain would pivot at it's connection at the tractor, so the PTO shaft also would not prevent a backwards flip.
We don't know if the chain was connected to a drawbar, or what type of drawbar if it was.
It could of been a cross-drawbar on the 3 point hitch that wouldn't act like a wheelie bar, and if raised too high would flip tractor backwards.
If it was a lower standard "swinging" drawbar, this
could act like a wheelie bar, but only:
1) If long enough,
2) If the ground under it was hard enough that the drawbar doesn't just dig in (i.e. drawbars aren't not wide enough to support a tractor, and not dig in on most soils)
3) If tractor has enough upward momentum it could overcome any wheelie bar effect (if there is any).
More speculation. Doing a job like this alone, is much more dangerous and prone to freak accidents. That FACT is much more important than establishing the geometry and tractor weight.
Knowing the geometry is important.
It's only a FREAK accident if you don't know the geometry.
(To me) it's not an accident when someone hooks a chain at a point above the rear axle. The results are totally predictable, but to a lot of people it would be an "accident". (Again, we don't really know what happened).
It's like the other thread on TBN a few days ago from the guy who added a lot of weight ABOVE the center of gravity on his tractor then "accidently" rolled it over (on what looked like relatively flat ground) and has no clue what happened.