No idea from the reading why it started rolling, but it's a good reason to throw on the parking brake and chock your tires when you leave a vehicle like that.
I think you're being kind of unfair and a bit mean-spirited about it even if nobody reads this site. He was doing volunteer work, being a community asset. If you rented a snowplow to clear a grandma's driveway you'd be all over the place here talking about what a kind person you are.
Balaam described the road Renner was clearing as a private road, not maintained by the county.
Could have happened to anybody else and you'd probably be like "oh well, that's part of the job."