what's the criteria for "trending"?

Good question. A better System would help find new active communities for your interests and help the community rise even more. But right now its showing a lot of totally empty communitys.

I think there should be a lower Limit to Show under trendig. Something like 1 Post and few comments a week.

Perhaps it's counting the number of views and showing you what other people viewed recently. This one showed up on "hot" for me.