Did you look through the mods you have installed (as was mentioned above)? I know there's one that assigns the initial user group based on the email domain (and there are probably others that are similar). You could disable all plugins (by putting define('DISABLE_HOOKS', 1) in your config.php) and try a registration just as a test.
I assume you checked the promotions to make sure there isn't one promoting user there (yeah, it probably should get deleted or at least not be active if the usergroup is deleted, but maybe it didn't).
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