It's all a matter of salience. Simple minded white skinned people see one black skinned person behaving poorly and assume they all behave poorly. Simple minded black skinned people get called out, criticized, or harmed in any way by white skinned people and assume racism. Throw in bad experiences getting robbed, assaulted, pulled over by the police, being treated in a condescending way, and these people grow polarized. When a fracturing issue like Trayvon or Michael Brown comes up, these people are going to see what they want to see, and the drama ensues.
It's also what we're seeing with the whole SJW vs MRA mess, really. People were hurt, people know someone who was hurt, people heard a story about someone who was really hurt, and now they're out to hurt back. So many people in these groups have either been swindled out of their belongings and child custody or they're been abused in one form or another or they've been unfairly judged.
And then the media will fan the flames because that draws the attention they seek.