I had heard the drooping drawers look originated in prison. What was that all about?
According to guys I knew who BTDT (prison time), sagging originated because prison-issued attire wasn't all about the fit. Prison blues were either too tight/short or too loose/long. Most opted for loose/long.
Later it began to be a way for certain homosexuals to advertise their availability. Butt cleavage was attractive to some.
After that, as gangbanging became popular, gang members would dye their boxers according to affiliated colors (red=Blood, blue=Crip, etc.), let their pants sag enough to expose the boxers and flaunt their gang colors that way.
Finally, those inmate-inspired fashion trends made it to the street.
At least that's what I heard.