It's fairly straight forward. When did the tensions start?
Obama's Beer Summit - a Harvard black professor is arrested by a white police officer because the professor tried to break into his own house and subsequently lashed out at the police that were called. He claimed this was every day racism (which wasn't supported by any stats or facts).
A few other issues later, where everyone that opposed Obama was called a racist.
Trayvon Martin, later ruled by a judge and jury to be a criminal that assaulted a Hispanic neighborhood watchman and got shot in the altercation, Obama told them "Trayvon could've been my son".
Police got shot by a (black) sniper in an ambush, Obama delivered the eulogy and instead of condemning the violence, he expressed "the need to understand the grievances of African-Americans who’ve protested racial bias"
That's when relations between blacks, police and whites started to decay rapidly. Many more occurrences where Obama should've called for peace and reflection, and condemn gross criminals (Michael Brown comes to mind) and instead he called for hatred, divisiveness and outright race wars.