What drives the "trending posts" list?

[Falco]

If you mean Top when you say trending, this is a good read:

Improving the "Top" criteria

The top page in Discourse is intended for two cases It is where new users are directed after they join the site, and where existing users are directed after long absences from the site. The idea is that we show people the “greatest hits” of the community when they arrive, whether it is the last year, or the last month. When you want to catch up with a busy Discourse and skim just the most interesting / useful / active topics. Sort of like the summarize topic option, but at the topic list l…