I'm not sure where David's information comes from but I do know that the Bridge Run calendar many times has the wrong information on it - sometimes they list races that don't happen any longer and sometimes they list races on the wrong day.
The Running Club's race calendar has information that is either given to us directly by the race organizers or I've found it on another website such as Active.com or ActionCarolina.com.
As to your questions - why so many on the same day? Who knows why some race directors don't move their race so they lessen the competition?