The images accompanying the story show that the Roll-Over Protection System (ROPS) was inactive and locked in the "down" position.

These mowers are made by Excel. In the manual for my Excel mower (similar design but sold under another brand) not only do they warn of steep slopes like those shown in the photo, but they also warn to ALWAYS operate the mower with the ROPS locked in the up position except for temporarily lowering it to get under tree branches or other low-hanging obstacles.

Most of the time I see mowers equipped with ROPS, the operators have it locked in the down position.