It could be spam bots registering on the site. Take a look at the user names and email addresses they're using. If the user names look like a bunch of random letters and numbers and the email addresses are from free email sites, they are more likely to be bots.
It is a creepy feeling to have a bunch of such registrations, though if you keep your WordPress installation upgraded and use secure user names (not "admin" for the administrator account, for example) and passwords, you're probably okay. You are running the kind of site that is somewhat of a lightning rod for attacks. I would certainly have a backup routine in place for the site files and database so that if you do get hacked, you will be in a position to rapidly put things back to rights.
You can try a plugin like Stop Spammer Registrations to try to prevent them from registering. It sure would be a good idea for a potentially controversial site.