When Really Simple SSL and WordPress MU Domain Mapping both are enabled, non-default domains are first redirected to SSL and then redirected to the default domain. When there is no SSL certificate exists for the non-default domain, the browser generates an error and the redirection stops.
This plugin disables Really Simple SSL for the non-default domains. So http://nondefault.example is first redirected to http://default.example, which is than redirected to https://default.example.