I think it’s a reference to r/fffffffuuuuuuuuuuuu a very old, established subreddit for rage comics memes. Then when the trans meme subreddit became a thing, they copied the style of the name. Then they brought the same name over to Lemmy.

There are other subreddits with the same style name, like r/iiiiiiitttttttttttt for IT related memes.

I could be wrong, I wasn’t there for the formation of the /r/traa subreddit, but that’s my speculation.