In Will Pfeifer's run, half of San Diego was sunk during a mysterious disaster. A portion of the people living in that portion of San Diego survived and that part of the city became Sub Diego.
In the artwork for those issues, the convention center, which was apparently holding Comicon at the same time, was part of the section of San Diego that sunk, and thus part of Sub Diego.
In the article by the DC guys, the Sub Diego folks are still putting on Comicon.