My guess is he's an O5 that didn't make O-6 (normally about 21-22 year point), so hanging around only gets him small incremental bumps in his pension. He can retire and work for a defense contractor and easily double his current active duty pay as well as start up a 401K.
Those guessing general officers are way off...with their base pay + extra years service they knock down around $10k or more in retirement.
See my 170