To be a "nuclear qualified" naval officer requires a pretty high IQ to begin with, and years of school and experience. With a booming high-tech economy out there, these "nukes" have better economic opportunities as civilians. Lower ranking nukes are seeing the same opportunities, and the navy has a bonus program for them as well.
The U.S. Navy has a shortage of Captains (the naval equivalent of Colonels), who are qualified to command a nuclear powered vessel. The navy wants to have 130 of these captains on active duty (90 for subs, 40 for surface ships), but is currently 28 percent short. So the annual bonus paid to nuclear qualified captains will be increased from $12,500 to $22,000. That is expected to induce more captains to stay on active service, and not retire. It's the unusually high rate of retirements over the last few years that has caused the shortage. Officers are eligible for retirement at half pay after twenty years of service, or, after 30 years, they can retire at 75 percent pay.