I have setup the scm plugin with the credentials in the sonar configuration. When I now run sonar:sonar on a different machine, the credentials on this local machine in folder $HOME/.subversion/auth will be overwritten with the credentials from the sonar server. You can simply discover this by:
1) move $HOME/.subversion/auth to $HOME/.subversion/copy_auth
2) execute mvn clean install sonar:sonar
3) check $HOME/.subversion/
You will recognise, that the auth folder will be created from the scm plugin with the credentials from the sonar server.