Could also be due to the disproportionally high rates of crime among black men. That should never be an excuse for prejudice, however.
https://ucr.fbi.gov/crime-in-the-u.s/2012/crime-in-the-u.s.-... https://infogr.am/Black-34991937313
https://ucr.fbi.gov/crime-in-the-u.s/2012/crime-in-the-u.s.-... https://infogr.am/Black-34991937313
[1] http://www.apa.org/news/press/releases/2017/03/black-men-thr...