All spam bots. 
This is most likely the case. 
I've been banning lots of these lately. How are they defeating the captcha?
There are a ton of bots that automate it, on the other site we were getting 10+ bots register each day, with spam sigs and links.
Only way around it I found is to enable a security question in sign up like
"Who was the worst and ugliest President of the USA in the history of the universe?"
Dont put the answer in the field or the bot will read it and answer it.