I have been through this three times in the last two weeks and the problem is that there is one metal detector/x-ray machine for this line (the one on the Delta/NWA side of the terminal. What happens is that the regular line feeds into the area very close to the premium x-ray machine and folks from the plebeian line look to their right and see a short line - NO TSA or other security person to keep them away and it goes to HELL in a hand basket.
Not sure how bad it is on the UA end but usually they controller it better.