IMHO what started it was the Nation of Islam and their propagation of the idea that Christianity is "too white", that it's contaminated by the devil-essence of the white man. The significance of the "X" is that it cancels out the white slave name and replaces it with nothingness; it's a symbolic act of annihilation both of the white race and of whiteness, i.e., everything that European civilization has ever done or ever stood for. Most people have no idea how overtly racist the Nation of Islam was in the 60's. They've toned it down in the last 25 years, but not by much.
Not that they have anything to worry about, with their liberal media buddies covering for them -- unlike Baptist preachers who preach against sodomy.