To add to this - it seems to change depending on which sub-forum you're looking at.
As I type this in Meta, for example, the "trending threads" listed to the right all look like they're from the very start-up of ENWorld; each is dated Jan 8 2002, and the topics suggest they're all from the then-equivalent of the Meta subforum.
But on the page I just came from (the D&D subforum) the trending threads seemed to make sense for what's currently "hot".