Just saw this story ^^^^^ today on the news and I said a prayer for the poor fellow who tumbled down the embankment when the mower either lost traction or maybe he turned too quickly. After a long span of dry weather, the area recently got a tremendous amount of rainfall.
Of course add the fact that the green winter type grass is slippery enough, even on a flat surface and the turf tires on the mower didn't help either.
If the news link would show the picture or video of just how steep the location is, it would shed more light on the story. The area is very familiar to me, as it's real close to an airport where an acquaintance of my family owned a flight school.
I have mowed some very steep areas and have had at least one close call before, but took evasive action and avoided injury or damage to my machines.
Be very careful if you have to mow steep places or do any other type of work in these type activities/tasks. A rollover can happen so quick.