Why is my white hair turning brown again?

When I was about 40 years old, my brown hair started turning white; not gray, but white. By age 58, my hair was almost completely white. But in the past year (65 years old), brown hair has started growing in. It’s still white on the top, but the brown is coming in, turning everything gray. WHY?? My medications haven’t changed; no new products. No one I talk to has ever heard of this.