A cop arresting George Floyd for passing a counterfeit $20 and resisting arrest put his knee on the neck of George (in a move that is legal in Minnesota, but illegal in some other states). George complained he couldn't breathe, and after ignoring his complaints for several minutes, George died. Since George was black and the officer was white, the far left activists and their media race baited (and dog whistled) their violent wing (Antifa, BLM, etc), so we got violent riots and looting that had virtually nothing to do with the injustice against George. Unlike other Fake victims (Trayvon Martin, Alton Sterling, Micah Johnson, Eric Garner, Michael Brown, OJ Simpson, Philando Castile, Rodney King, Sandra Bland, etc), this one was on video and more clearly the cops abuse of power. While there was no evidence this was race based, the public was outraged over the unnecessary continued abuse of a subdued suspect and justice was going to happen... in fact the officer was arrested for 3rd degree murder and the riots and looting continued. Obama and other far lefties that had previously said looting was not about the crime but opportunism, of course egged the rioters on. If you believe the Democrats/Far Left rhetoric, this display of anti-Americanism, was inevitable.
Derek Chauvin was the cop that arrested (and killed) George Floyd. It turns out they worked at the same nightclub (El Nuevo Rodeo Club) for at least a year, but we don't know if they knew each other. (Either is possible). 
Far Left Dogma
If you believe the far left, then this was inevitable. The left has been preaching for decades that:
- Minorities never had any opportunities because the system was out to get you
- The police only ever abused minorities, and virtually always got away with it
- Your station (Poverty) meant you'd never get justice without violence
Never mind that none of that is actually true. The far left still believes that in the country with the most opportunities for minorities in the world, is one of the worst in the world for oppression. It doesn't have to be real, they just have to believe it.
So those that believed that left, were outraged, other opportunists and sociopaths on the far left knew from prior riots (and tolerance/sympathy towards them) that they could go out and rage and steal and nothing bad would happen. Weakness is provocative, and some are shocked that they got what they encouraged. They said, "Act out, we won't punish you... we'll make you famous and make excuses (and blame the system) for your behavior", and low and behold, some listened.
So this happened because of far left dogma, that convinced some that they had no opportunities for justice, or others that there should be no consequences for bad behavior because tearing America down is the only ay to get Social Justice. Either way, you'll notice this usually happens by the Democrat base (young dumb kids and disenfranchised minorities that are Democrats strongest voting block), and in Democrat strongholds (inner cities), and they were repeating rhetoric that is prevalent in the DNC and their media. But they're shocked that this happened, and want to blame Republicans for it.