It doesn't appear to be the result of following a link with a specific URL. If I open a new session of Chrome, go to the main giantitp.com page, and click on the sidebar link to the forum, the URL in my browser is
Code:
https://forums.giantitp.com/
but I am still given the mobile skin. As I said, it isn't a big deal because I can work around it by using a desktop UA.