I was referring to the upcoming model. Only 1500s are available at the moment. The Suburban 2500 is returning for 2016 (model year) for fleet only: See link
2016 Chevrolet Suburban HD will be 3/4 Ton; Fleet Only
The next Expedition will adopt the new towing standards set forth by the SAE. Either way, Ford does test and evaluate the towing performance of its trucks and SUVs. They are not arbitrarily assigned tow ratings.
Toyota adopted SAE J2807 around 2013. GM only started using SAE J2807 for its 2015 models. So GM has not been using it for a couple of years as you stated. Ford also adopted SAE J2807 for the 2015 model year.
Actually, the "bump" in towing capacity was small and not praiseworthy. Therefore Ford did not push to advertise the change.
Expedition EL 4x4 saw a minor increase by 400 lbs;
Expedition EL 4x2 saw a 300 lb increase;
Expedition 4x4 saw a 200 lb increase;
Expedition 4x2 saw no change. Nothing to write home about.
The rating change stems from the extra 55 lb-ft of torque the EcoBoost added (and at lower RPMs, then the 5.4 Triton was capable of), remapped electronic throttle management, and design changes to the Expedition's T1 frame (T1 platform), which can be seen by a trained eye, such as a additional structural cross-member.
These enhancements help the heavier models move more weight faster (thus offsetting their weight penalty) and translate to the slight "bump" in ratings. The disc brakes have always been rated for GCVW over 15,000 lbs.