Outside of the 6 nations banned, it could just be anybody else at random really. CBP officers have infinite authority to reject people at will. It's commonplace at the Canadian borders for Canadian citizens to get turned away[1] [2], I'd like to point at their skin colour but then I'd get called hysterical.

[1]: http://www.cbc.ca/news/canada/montreal/canadian-woman-turned... [2]: https://news.vice.com/story/another-canadian-of-moroccan-des...

P.S: CBP officers have a difficult job so the decentralised decision-making is understandable to a certain degree