So I googled a few sources, blogs, research, definitions and chat board discussions. From which I pulled these tidbits of testimony to the trend.
Wikipedia (how I love you) Says
Origin - Lee D. Baker, Dean of Academic Affairs at Duke University, states that it is widely believed that sagging was adopted from the United States prison system where belts are prohibited.Belts are sometimes prohibited to keep prisoners from using them as weapons or in committing suicide by hanging themselves. The style was later popularized by hip-hop It has since become a symbol of freedom and cultural awareness among many youths or a symbol of their rejection of the values of mainstream society. artists in the 1990s.Sandra E Graham also agrees with this definition in her post
The Origin and Purpose for the Baggy, Saggy Pants (Mens/Boys) Fashion
And this special Chart depicting fines for saggy pants:
And a past Inmate tells us here -Answers.com - that "sagging" in fact has nothing to do with the belt, and everything to do with being "available" to other men.
On Snopes.com sagging is much further defined and researched, to include both theories of sagging stemming from the badly fashioned prison garb and the gay prison population expressing availability.
So them's the loose facts of the day kids. I learned something new!