Nintendo has filed a trademark for the term Hyrule in Japan, Europe, and the United States. The first thing that comes to mind is that Nintendo is just trying to protect Hyrule Warriors, but the trademark would not actually protect the Japanese version of the game and the trademarks are not currently active, suggesting it could be for future ideas. There appears to be a hiccup with it in the United States, but they are working through it currently.
The real question here is why they are trademarking…