IIRC that's been a thing since the '60s, when the term Black began to replace Negro in popular culture. It seems retrograde to us now, but even capitalizing Negro was controversial in the '20s.

[Negro itself had become an unpopular term, associated with a subservient type of Black person, one whose politics were more about patience instead of protest. The 1960s ushered in the Black power movement, inspiring a generation to claim that which had been demonized. In the late 1980s, Jesse Jackson pushed “African-American” into common usage, offering a new term that wasn’t tainted by a racist history — and conferred the respect of indisputable capital letters.

By 2000, Black Americans had a choice of what to call themselves on the census: “Black, African Am., or Negro” (because some older people preferred it). All racial and ethnic categories are capitalized on the census — including White.](http://www.nytimes.com/2014/11/19/opinion/the-case-for-black-with-a-capital-b.html?_r=0)