I've been wondering about that tab... In the end of 2015, YouTube made a blog post related to their YouTube Rewind 2015, and their new trending tab.. They wrote a blurb about what they thought was "trending" in 2015: "Celebrate some of your favorite moments on YouTube through our official top trending videos of 2015, based on the amount of time people spent watching, sharing, commenting, liking, and more."
I guess the "and more" part lets them be extra ambiguous because there isn't anything else left on their list that viewers can do with a video. o_0 And we don't know if the criteria they had for trending videos in 2015 is the same as the criteria they're using with their trending tab...they just mention them in the same blog post...
Then they say at the bottom of their post, "...today we unveil the new YouTube Trending Tab. This new tab in your YouTube app delivers the top trending videos directly to your Android, iOS, and desktop device. It's the best way to catch the videos, creators, and trends that people watch, share, and talk about each and every day. See 'em as they take off and before they appear in YouTube Rewind 2016."
I guess maybe it's supposed to be a viral video detector of some sort? My trending tab is filled with American stuff -- I wonder if the trending tab is different based on what country you live in?