To me it looks like some popular spamming software (like thebestspinner, etc) just integrated you and now everyone who is the software is now hitting your site.

To me it looks like some popular spamming software (like thebestspinner, etc) just integrated you and now everyone who is the software is now hitting your site. The good news in this case is that's it'll be easy to spot the pattern and block it, the bad news is you're entering a never-ending cat and mouse game.