How can I include Profiles on Drupal user registration?
My guess is the problem is with Multiple Register since many folks have integrated LoginToboggin and CiviCRM. Turn off all of the three Drupal modules you mention, then reenable them one at a time to isolate which is causing the problem.

You can probably benefit by examining the source of the page with and without the troublesome module enabled in your browser using Firebug or something similar. You may end up needing to provide a patch to Multiple Register or making a custom extension in CiviCRM to help CiviCRM and Multiple Register play nicely with each other.