Yep, both PGPhotographic and Nixie nail it as to why we have a large following from Scotland. We initially had a large amount of Scottish models, and thus it was easy for the site to attract Scottish photographers when it was opened up to photographers several years ago.
The site has been growing a lot down south especially in the last 6 months. As more people join, they recommend it (hopefully! :P) to their friends, who then join, who then recommend it to theirs etc etc..
James