I'm not really sure about ATL curbside, but CVG has to put the bags on a cart and take them to a different belt. It may appear to be backed up, but they stack them and take a bunch at once. This is normal procedure. In CVG, they have to use a belt that goes to TSA. Maybe it's possible the belt you saw not running may not be used for that. Another possibilty is the belt
may have been shut down by the TSA. They do that to us often when they
start falling behind. This results in us having tons of bags waiting around.

Just my two cents. Like I said, I'm not familiar with ATL curbside, but thought I would try to help.