I agree that it's most likely SSSS. I've seen it before on UA, AA, DL, BA and OZ and virtually every time I'm actually checked in but can't get a BP until I see somebody. An unfortunate side effect seems to be that I can't get a mobile BP on any of the connections as well as the original departure. IAH TSA wanted to send me through regular screening because of SSSS even with Pre-Check on the BP and my Navy ID. Supervisor took me over the Pre-Check and the computer said "You're good to go" and I was sent through Pre-Check. Supervisor didn't seem too happy with the initial screener rejecting my BP without scanning, since things were already backed up due fairly long slow lines and they didn't want to add any more load to the regular lines when Trusted Travelers aren't selected for extra security.