USATF rules state: "Any competitor or participant jostling, running across, or obstructing another competitor or participant so as to impede his or her progress shall be liable to disqualification in that event."
The "liable" leaves some wiggle room. That said, you would think an athlete who, during an XC race, threw another athlete down to the ground hard enough to cause a DNF would get disqualified. Maybe not right away, if no officials saw it happen - but certainly after he admitted that he did it, as Nate Jenkins has on his blog.
Here is an excerpt from his latest post:
http://nateruns.blogspot.com/2016/12/weekly-training-blog-december.html
"Now the story…. So let me be clear. I started it. I was moving up through the field and after 2 miles I was cutting across on the tangent and I cut a guy off. Not on purpose but I was cutting across the field and there was contact and we both got a good stumble in. A few seconds later the other guy rushes up next to me and gives me a good elbow. Ok fair enough. Then he keeps leaning into me and elbow pushing me over to the edge of the course, which at this spot was some heavy bushes. I probably should have said something but I didn't. I have a temper when I race, and yes I know I was in the middle of the pack with no shot at doing anything but that doesn't have any impact on my competitiveness. Anyway I sort of used my right arm and threw the guy to the ground. That wasn't really my intent. I just wanted to give him a good enough jolt that he would cut the shit and we'd move on but down he went. Now his teammates around him were none too happy, for obvious reasons and were shouting and stuff. I was moving faster so I was out of range fairly quickly but when someone tells you I'm an a-hole, keep in mind sometimes I'm an a-hole. I actually sort of figured I'd be dq'd but either they never ended up complaining to the officials or the officials were like Umm you all finished a 100 and what? and blew them off."
Nate should be given credit for being straightforward about what happened and for his seemingly honest description, though I think this part of - "I sort of used my right arm" - is perhaps a little understated.
If Walter Sobchak were a USATF referee, you are god damn right we wouldn't even have to start this thread!