Similar to Mastodon's spikes last year, it seems. Anyways, there is data to think about. Source
There are also conscious efforts to weed out bots and other measures that try to remove potential cancer from spreading.
There was a post recently that outlined bot weeding efforts on a couple dozen instances that tanked user number by something like 1/5 - clearly visible on graphs.
Lemmy’s doing great. Even if plenty small communities are still not big enough here.