Tracked vehicles - bulldozers or sno-cats - are sketchy to operate on a slope. They can climb straight-up or descend straight down real well, but operating in "side-slope" conditions is genuinely scary. Those grousers on the tracks make great "runners", and the wider the tracks, the quicker-better the slide, especially if the snow is packed.
It is possible that a snow-cat parked sideways on a slope with engine off and blade down and parking-brake set will just "settle" and start to slide sideways down the hill all on its own. If you are on the down-hill side of that, and the heavy machine slides over you, you are hurt. Or worse.
I've seen photos/report of that happening with a D6R LGP (some 50klbs) on about a 30% side-grade, the operator was just getting off to adjust blocks/ramps to walk it up on a low-bed, and the thing took off down the side-slope. Thankfully, the operator was dismounting on the up-hill-side and just leaped the last couple feet to the ground and watched it slide sideways to the bottom of the slope. Anyone on the downhill side of that slide would be hamburger.
Is it possible that Mr Renner was on the downhill of a sideways slide? Yes. Even a few feet of sideways slide would have enough momentum from a heavy machine to inflict injury like Mr. Renner's.
A tracked machine is a LOT of rolling resistance, and for the most part does not just "take off" moving unexpectedly. Operators are aware of that. If a side-slide is what happened, Mr. Renner would not be the first (wily veteran or not) to have not recognized that hazard.
