I don't know, I can only speculate.
A lot of talk about heredity. In my case it does not serve, for there is another patient with vitiligo in my family.
Food and environment, for me, also are not justifications.
Shocks and emotional can be a trigger for the emergence of the first stain: watch a strong scene of violence/accident, to lose someone, end a relationship, and etc. If this is it? I think that the science, as well as I, also, can only speculate.