I read somewhere where it is a GANGSTA type of thing whereas,
when you get thrown in jail, they take your belt away
so you won't hang yourself.
So these stupid assed kids try and replicate that thing.
My son came home "sagging" one day with some specially
tailored sagging jeans. When he went to bed, I snuck into
his room, and threw his "sagging jeans" into a roaring
fireplace!

I think he got the hint.