It's the pendulum swing of pretty much every community on Reddit.
- Community starts out with a small group of users dedicated to quality content related to the topic
- Community growth reaches a point where the most popular posts begin to trend outside of the community
- New users join the community after seeing popular posts show up in their own feeds. Growth accelerates
- Community becomes "popular" enough that posts regularly trend outside of the community
- New users flood in
- Users flood the community with low-effort content to karma farm
- Community now sucks.
It happened to basically every big sub on Reddit once reaching a large enough size.