-snip-
Unfortunately, there's no info in the 'Trending' page nor in their FAQs about the requirements for getting into the 'Trending Cryptocurrencies'.
I doubt that the cmc team will explain the algorithm to be trending, because after all it has to be purely generated.
Better than asking here in the forum where there's no official CMC representative.
Users will most likely answer with guesses and other non-related stuffs.