It is probably due to a number of people stopping using their alts after some instance hopping.
Also a few people who came to see how it was, and weren't attracted enough to become regular visitors.
Curious to see at which number we'll stabilize.
Next peak will probably happen after either major features release (e.g. exhaustive mod tools allowing reluctant communities to move from Reddit) or the next Reddit fuck up (e.g. removing old.reddit)
Yeah, there's even upcoming third-party apps to fill the void left by apps like Apollo (at least for a while).
What keeps me there and not here are some niche subs for music genres/subgenres and bands, and I don't see any of them crossing over to the fediverse until Reddit completely and utterly dies. It even seems like some went over to Discord and I'm not really a fan of that, but it's probably more lively there than here for my interests.