For a long time there have been spammers scraping in search of email addresses to spam. There are all kinds of scraper bots with unknown purpose. It's the aggregate of all of them hitting your server, potentially several at the same time.
When I worked at Wikimedia (so ending ~4 years ago) we had several incidents of bots getting lost in a maze of links within our source repository browser (Phabricator) which could account for > 50% of the load on some pretty powerful Phabricator servers (Something like 96 cores, 512GB RAM). This happened despite having those URLs excluded via robots.txt and implementing some rudimentary request throttling. The scrapers were using lots of different IPs simultaneously and they did not seem to respect any kind of sane rate limits. If googlebot and one or two other scrapers hit at the same time it was enough to cause an outage or at least seriously degrade performance.
Eventually we got better at rate limiting and put more URLs behind authentication but it wasn't an ideal situation and would have been quite difficult to deal with had we been much more resource-constrained or less technically capable.