What determines how you get onto trending? Is it comments? Views? Ratings? Favorites?
The trending stories are always one that experienced the sudden surge of activity in roughly the last 24 hours.
The overall rating doesn't have any significant effect on it, as occasionally even the stories with "bad" rating could shoot up the list.
Followers / readers could be the factor, especially the "reading" or "plan to read" could be accounted for.
We ultimately don't know because the site keeps it secret to avoid people gaming it, but we could rule out certain factors like comments.
It, however, works differently than the Royal Road's trending.
The Royal Road's trending is the hidden list, based on growth (in number of followers). If you steadily grow the number of followers, and update slowly, let's say one per a week, you will be stuck in their trending list, which is mutually exclusive from the Rising Stars. It's one of the most static list on that site.
We could verify it isn't the case, any story could enter the Scribble Hub's trending in any time if it caught enough interest.