The saggy pants trend began with prison inmates who many were given
pairs of pants that didn't fit and sagged all around their waist. Later,
rap artists picked up on this trend as a form of respect for those
inmates and became extremely popular in pop culture.
However, the trend Justin Bieber is using where the pants sag
only in the back with the rear end plainly hanging out had a whole
different meaning with prison inmates. Those with saggy pants who pulled
their pants down in the back was a signal from inmates that they were
sexually available and willing to be "girl friend" or the bottom to
other prisoners.
Another term given to it is PBS (the Prison Bitch Syndrome)
It would seem odd that Justin Bieber could not know about this for as long as he has
been doing the trend. At some point, it would have been brought to his
attention.