Timing and request patterns. The increase in traffic coincided with the increase in AI in the marketplace. Before, we'd get hit by bots in waves and we'd just suck it up for a day. Now it's constant. The request patterns are deep deep solr requests, with far more filters than any human would ever use. These are expensive requests and the results aren't any more informative that just scooping up the nicely formatted EAD/XML finding aids we provide.
And, TBH, I don't care if it's AI. I care that it's rude. If the bots respected robots.txt then I'd be fine with them. They don't and they break stuff for actual researchers.