> I wonder what factors might cause this reversal.

Very likely: Stress. Also diet and exercise. They are interrelated. But for people that I’ve known have hair go gray and back again, it was stress.

I know this is anecdotal but I know someone that experienced / is experiencing a similar thing. Several silvery hairs reverting back to pigmented after leaving an overly stressful work environment. She only had a few, but of those few perhaps 30%-50% have reverted.