Can confirm it is indeed due to an upsurge in bot activity.
They are currently wreaking havoc with the registration system. Seems that quite suddenly they can all get around the captcha (jumbled lettering) even though I have it on the highest setting.
Overnight 20 new accounts were created. 19 spam and one real one. Bans are not totally effective at blocking them because they are able to use so many different IPs, servers, and email addresses.
I am seriously thinking about disabling the registration form permanently and asking people to write to me, personally, if they want to join the forum because the automated sign-ups have become so prevalent.