Trending posts are the ones that have consistently the most user engagement (upvotes and comments). Naturally, more upvotes will increase the payout (as well as the likelihood of a large payout), so it tends to go hand in hand with a large payout. If you noticed, the number of replies for each topic is extremely high. It's a combination of factors. I did a quick search and didn't find any satisfactory articles that covered the specific algorithm that determines it, but you could try the help desk on Steemit.chat and see if you can get some answers there. I honestly don't know, otherwise I'd just tell you lol