Snowcats running during operating hours at a resort is pretty common place. Especially one the size of Mammoth. The take care to stay to the sides and give as wide a berth to skier traffic as possible. Of course that doesn't account for a jack ass snowcat operator or a jackass snowboarder.
Pretty steep price to pay either way. My hunch is that it was her fault, but there is no real way to tell by that article. Could easily be the other way around.