Tip.it's forum database was stolen not too long ago after the site was compromised by hackers. This included hashed passwords, IP addresses as well as emails used for registration. They could've gotten your account one of two ways: if you used the same password on Tip it as you did on Runescape, or they could've used the IP address to help recover your account. There are other possible ways of course, but considering you've been a member of the community for so long, it's definitely a possibility.