How a song gets trending ?

[[Member Nare…]]

I wonder what is the smule system of trending a song ? 

[[Member open…]]

As with any social media platform, they won’t publish details about that, as it would make “gaming the system” easier. 

I also wonder why you are asking. Appearing in the “trending” section of Smule doesn’t really mean much and you will see that it won’t necessarily lead to much views, let alone new followers. 
Most singers will usually grow their account organically. Good collabs attract activity such as likes and comments, which then can appear on the homepage of connected users and so on. It’s about the collaborations, not the algorithm.