Perhaps its a side effect of AA cozying up to Citi (e.g. Citi demanding to be the sole lounge-access credit card partner)... same way UA is with Chase, and DL with Amex.
On a side note, a popular boarding area blog reports that Amex will open its own Centurion lounges at LGA and SFO (LGA being outside security, no idea where)
http://thepointsguy.com/2013/12/amex...r-lga-and-sfo/