In my experience the LowSodium subreddits pop up when the main one has become so filled with complaining that you can't have normal discussion about the topic. For example, in a subreddit dedicated to a game, there may be a bunch of people filling the place up with outrage over some change or something. Outrage generates a lot of upvotes, so those become most visible. You also get a lot of people who think that their special snowflake opinion deserves its own post, instead of just commenting in a related thread. But there are people playing that game who need help with a strategy or some other game mechanic, or want to talk about some mod they found, or just want to share an experience. etc.
If the main subreddit becomes impractical for those sorts of posts, then another one will pop up.
"It's like you have a neon sign that just says 'raw dog me! I'm a bottom!'" -Queen Maeve