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.