Dan,
Showcase for the most part is not about "bumping" or being "popular".
I feel that Showcase is a place were people share their progress, post new tunes that they created with assistance of BIAB, exchange opinions, ask for specific advice, make friends, etc.
Some people are more active than others, they write more music, listen and respond to music other people post...that is why some posts seemed to be more "bumped" than others.
About the genre thing. Most of us will listen to a genre(s) outside the scope of prime interest just to expierence and explore something new, support artist, read stories, lyrics, learn about methods used etc.
Having said that... Perhaps it is a good idea for a poster to be able to "tag" the genre their tune is in, that would most likely save time for PG staff. There is a way to sort by "genres" in showcase, but seems sorting needs minor improvements. I do not use it personally, but here it is:
https://www.pgmusic.com/user-showcase/