Some of the larger machines roll out a 1500 pound bale.The bale is ejected so it will clear the mechanism,so it has some motion which can start the bale rolling.Poor judgment to use such on steep hills,but it is mostly a labor(or lack of labor) issue.
When you are on an embankment with a round hay baler, you roll the hay up the embankment, always. Then when through making and wrapping the bale, the rule is to turn the rig sideways such that the bale is not oriented like a wheel heading down the hill, that is sideways. Then eject the bale. Green bales will sit, not roll. This was an accident. In the US, the farmer would lose his farm to a tort lawyer, who would win even on the basis of “what are the chances a bale would become airborne and leap over a hedgerow of trees and fencing and land precisely on the front roof of a van moving on a highway”
One other comment- it is haying time in England, fall having arrived for the last real “cut” tedding, raking and baling. Round bales were invented for ease and for preserving of total digested nutrient protein. As tight as one wants to get a square bale, they lose their TDN fairly quickly. In the states the “shelf life” of round bales is improved with waterproof covers wrapping them. This for the benefit of cattle, dairies, etc.