Any update on this one?
Other fora which show similar symptoms generally find that they are being hit by bots in their thousands trying to scrape their entire content, allegedly for AI training purposes. Trying to stop them often turns into a game of whack-a-mole, though black listing IP addresses which make more than a certain number of accesses in a specified period can be very effective. The latter requires that software is in place, which can be tricky if no-one has thought about its provision beforehand.
Good luck!
Paul