I just assume that what is trending in cmc is the most typed word in the search field during a certain period. However, the marketers I found on twitter use the reward method to perform certain tasks including adding their coin profile to the watchlist and some social media account boosts. And another fact, cmc also
collects visitor cookies.
If they will reveal the recipe to be trending, everyone will just game the system and we can see a lot of scam coins on the trending page. That is bad as that can miss lead the newbies. A coin should only be trending in a fair and natural way. On some sites like youtube and twitter for example they only way to get trending on them is when everyone is talking about them in the comments or by using a hashtag or by retweeting and watching their video's.
CMC has some similar functions like what we saw in social media so maybe it can also work the same but more importantly is when the coin is in demand. Conducting an airdrop, bounty and other forms of marketing helps too.
It is really likely the most typed and searched project and that's why it is trending. But there could be also other metrics and it's not just on CMC alone to figure out how to trend on their own platform.
For Twitter, it's a different way of making a topic get on trending and you're right with that. And in YT, it's a different thing on how a video gets trending and even there's no comments, it's reliant on the views.