Im not sure for coinmarketcap but with coingecko, if the star button on the project page got spammed or too many clicks on that particular page of coingecko tokens then it could show to trending. Ive see twitter influencers doing this and when Ive checkef the trending the coin/token is on the list already. But its not good if its due to that kind of trend where they do giveaway to do like or star but shoulf be based on actual view where the tokens get trendy in real form.