It seems like the whole feature is running wrong and should probably be hidden until it can be reexamined. Because every Lemmy server seems to have “trending communities” that are just empty new communities or are just single-user bot feeds. And those are fine uses of lemmy! But they’re not “trending”.
It’s what’s active in each instance. If they’re empty that means that instance isn’t creating any content. It’s also easily ignored. Different apps seem to handle this in different ways too and show completely different “trending” communities. Some handling it better than others. Also, if you’re the first one from your instance to visit the community… they’re going to appear empty. Subscribe to aggregate more activity.