I think she drove (or walked, mode of transportation is unspecified) across the border into Canada, was told her visa was inadequate for her planned activity and to return to the US to apply for a proper visa, when she did return to the US, she was detained.

The article is confusing though. This BBC article has a much clearer and more detailed timeline of her travels.

https://www.bbc.com/news/articles/c80y3yx1jdyo