"DiversityInc. has a column called “Ask the White Guy,” and in 2009 its author, Luke Visconti, explained “Why the ‘B’ in ‘Black’ Is Capitalized at DiversityInc.” His reasoning, in part, is that “Our capitalization of ‘Black’ is both a reflection of reality and of respect.”

[M]any Black people describe themselves simply as being “Black,” and this reality is reflected in a body of literature, music and academic study.

I do not believe “white” needs to be capitalized because people in the white majority don’t think of themselves in that way. I don’t think there’s anything wrong with this—it’s just how it is."

Quoted from here:

https://www.cjr.org/analysis/language_corner_1.php

Many people choose to capitalize Deaf, Blind, and Autistic for similar reasons - it is indicating that they identify with this aspect of who they are and are reclaiming it as an attribute they value. When I've seen white capitalized, it is usually people who are emphasizing whiteness in their identity, i.e. white supremacists.