There can't possibly be that many threads that qualify as "trending" that are getting deleted or merged. If a thread has enough activity to be trending, surely it is able to stand on its own. Is it possible that the trending threads algorithm just isn't regularly refreshing which is leaving some of these ghosts (and other older threads) on the page?
There are some trending threads on the page right now that have 7 or fewer posts in them over the course of a full day, there has to be something off in the algorithm.