Are you participating in the TSA PreCheck program (via Global Entry/NEXUS)?
Don't know how it works at ATL, but at DTW you need to go through the SkyPriority lane to get to the TSA PreCheck lane. Not sure how this is handled for non-status or Silver Medaillions unless Delta gives them SkyPriority access.
No, I'm not, though I'd like to. It's just completely random, which is why it's so baffling. Don't get me wrong - not complaining in the least, but I could really use that Sky Priority for those horrible LAS lines but never have it coming back.