If you need to fire a CB trigger externally you'll need to first load CBs API. See the below on how to do this.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18357-including-cb-api-for-usage-outside-of-cb
Next you just need to be sure the global $_PLUGINS is added to your function and call the trigger function after loading CBs plugins in. Example as follows.
Code:
global $_PLUGINS;
$_PLUGINS->loadPluginGroup( 'user' );
$_PLUGINS->trigger( 'TRIGGER_HERE', array( VARIABLES_HERE ) );
CB Auto Actions can work with Joomla events now though so you could optionally just use a Joomla event.