The Mongodb Watchdog Module is inserting watchdog events as mongodb collections. This is in addition to inserting the watchdog events as items within the designated collection (which is the expected behavior)...
This is the entry I added to the end of my settings.php file:
$conf['mongodb_watchdog'] = 'watchdog';
$conf['mongodb_watchdog_items'] = 15000;
This is the resulting collections added to the Mongodb Database (lower left corner you can see watchdog_event_7b8e.... etc):