He got a rare skin condition which made his skin get very sensitive to the sun. Eventually, his skin got paler and paler.
He was noticeably lighter when he released Bad when he was around 28/29 years old. By the time he released Dangerous in 1991 he was totally white, he was 33 years old.
Mid 1980s to the Early 1990s.
Because their mother was white.
Vitiligo