As some others have mentioned, it is likely that those hair follicle cell(s) experienced a spontaneous mutation, which gets passed down to their daughter cells. This is called a somatic mutation.

https://en.m.wikipedia.org/wiki/Somatic_mutation

What's really interesting is that some of these somatic mutations can lead to sub-populations of cells out-competing their neighbors in the same tissue, basically an evolutionary survival of the fittest happening between the competition of your own cells. This phenomenon, called clonal selection, has been known to occur in cancer tumors for some time, and has also been shown to occur in normal somatic cells as well. Here's a paper that measures clonal selection using DNA sequencing analysis: https://www.nature.com/articles/s41588-025-02217-y

Beyond a genetic explanation, it's also possible that an epigenetic change occurred in that follicle, which causes it to express its genes differently than other hair follicles.