I hypothesise that when a user makes a search, especially when drilled down into a specific category, the user wants quick access to the “New Topic” button, in case the search does not return the desired results.
This just happened to me when I was checking to see if a bug I wanted to report already existed. When my search turned out to be fruitless, the next natural step was to make a new topic, but in order to do that I had to jump back to the index page, click “New Topic” and select the Bugs category.
If you’re drilled down into a specific category, the category selector should default to that category.
Probably a good pull-request candidate?