I'm not going to say I'm an expert, but this what I have noticed: if your comic is trending at all within it's genre, it'll show up among one of the categories.
Let's check out the Mystery genre:
Take Line's Art-- it's currently in Popular and Trending... possibly someone (or a few people) have sub'd and faved a whole lot of episodes. You can see it wasn't recently uploaded (other wise it also appear in Fresh-- or maybe other comics have updated taking over Line's Art Fresh ranking). Depending on circumstances, a comic can appear in Popular & Trending, regardless of when they updated last because of viewer activity.
I haven't updated in a month and have appeared in Popular & Trending because of viewer activity (subs, comments, view and favs). Someone can find your comic from the forum, like/sub, and then boost you into the category where others can find you. If someone faved or sub'd when you were going through a quiet time, it can briefly put you in Pop/Trend.