Right, it’s not that there is something actively pushing hexbear to them, but because there are fewer places for them to go, it becomes more likely that they will end up in the remaining instances.
For example - let’s say 2 hexbear users like to look at memes. They were previously using the lemmy world meme community, but now, since they are defederated from there, they will go to different instances’ meme communities. But then, more defererate, and it funnels a higher concentration of them into the remaining instances’ meme communities.