For mander.xyz it has been bot scrapers. That time that you are mentioning it was scrapping via the onion front end that I am hosting for easier access over Tor. Yesterday an army of bots scraping via Alibaba cloud servers made the server unusable for a few minutes. The instance would receive a bunch of requests from the same IP range (47.79.0.0/16), and denying that full IP range fixed the problem.

Some instances implement anti-bot measures. For example, sopuli.xyz makes use of Anubis. I think that instances behind Cloudfare get some protection too. I am considering using Anubis for mander.xyz, but for now I have just been dealing with this manually as it does not happen too often.