New user registration issue, they get redirected to root domain rather than sub-domain

I have an MU site hosted on Hostgator. example: root site bubba.net, sub-domain workshop.bubba.net. Domain name http://www.workshop.com is mapped to workshop.bubba.net. But when a new user attempts to register, user gets redirected to bubba.net. What am i missing or is this is the way its suppose to work?