I sent an email to AFPC asking for more information and this is what I got back in reply. I retired last year from the U-2 and unlike what WarEagle thinks, the U-2 is a great job.

"If you are receiving this e-mail, it means that you have expressed
interest in possible voluntary recall to active duty by providing your
contact information and comments. The purpose of this e-mail is to
provide you with what limited information is currently available
regarding this recall.

Due to the rapid expansion of unmanned aircraft systems and other
emerging mission/rated requirement sets which directly support GWOT,
demand for experienced, rated officers is greatly exceeding current
inventories. There are many different options the Air Force is
considering implementing to help lighten the burden our rated force
currently carries, one of which is a voluntary recall to active duty.
For your information, a proposed recall package is in coordination, the
details of which are still being worked. Further information will be
released as it becomes available.

Please stay tuned, and thank you for your interest."