The most common cause for unauthorised account access is because your login details have been exposed on another site or are being used maliciously by the webmaster of another site where you've registered with the same details.
For example, don't register on one review site or forum with the same set of details that you register with at AdultWork.com. If that site gets hacked or if the webmaster feels like it, he can probably get your details from his own system and try them here.
Don't register on any site with the same password. These two sites will tell you if your email address is linked to any data breaches. Check both.
You can change your password for AdultWork.com here.