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Did he curb his wheels properly?
I joke, but that's why you're supposed to turn your wheels - in case the car "pops" out of gear. Not that a snowcat has wheels... But the point is that we curb our wheels (and it's the law) because the machines are mechanical devices (even well maintained ones) that are susceptible to failure.
Washoe County Sheriff Darin Balaam said on Tuesday during a press conference that Renner was run over by his snowcat, a device to remove snow, after the machinery started to roll away when he was not in the driver’s seat. He was injured during an attempt to get back into the snowcat, Balaam said. >

It was being reported initially as a safety system failure by at least two news outlets. I am not seeing that dialog now. >

That could still be true - sounds like it started moving without a driver.  Maybe a break failure. >

Or he didn't engage it fully. We just don't know.