It could have to do with the NSA's August 2015 plan for transitioning to quantum-resistant algorithms. In their new Commercial National Security Algorithm (CNSA) Suite, they advise a minimum 3072-bit RSA modulus:
(Doh... I don't know why I'm getting an invalid certificate authority error when trying to access that site, but Qualys SSL Labs confirms it's a real error. Yikes.)
They're DOD so why not. There's a lot of them that do. What confused me was that browsers in US didnt trust DOD PKI... probably quite reliable... while they have plenty of shady, less-secure CA's on their list.
For anyone reading the thread, this is an absolute must-read paper if you're at all interested in the near-future of production cryptosystems in high-risk settings.
https://www.iad.gov/iad/programs/iad-initiatives/cnsa-suite....
(Doh... I don't know why I'm getting an invalid certificate authority error when trying to access that site, but Qualys SSL Labs confirms it's a real error. Yikes.)