I can only think of one cause myself:
The machinery being pulled is weighing down on the back of the tractor, and circumstances happen where that force is magnified, perhaps by a bumpy ride with the machinery bouncing up and down on the hitch.
I don't know much about farm equipment, so this may or may not be correct at all.
I'm not sure what bm0p700f is saying. I don't see how that could result in the tractor flipping backwards.