It's fallout from global colonization..
It's part of the mental illness we all (Black folks) have inherited from that system..
Not just in South Africa: before the Black Consciousness & Black is Beautiful movement spread from AfricanAmericans to the islands, light-skin folks were highly insulted if you called them black, and even worse if you referred to them as African; Particularly in Haiti & Jamaica.

LOL.. after apartheid came down and the ANC took over, lots of "coloreds" started identifying as black to take advantage of Affirmative Action & some land redistribution programs. Opportunistic bastards!