When does it stop being an unfortunate accident and start being a crime? How many incidents does it take before we can agree on the existence of a pattern of abuse against visible minorities?

Each incident is itself an unfortunate accident. Together though, I argue they already are indicative of racial profiling, racism, and an a pattern of problematic behaviour among law enforcement professionals.

The death was an unfortunate accident, but the circumstances in which it occurred were not an accident. It is the circumstances that people are protesting.

By analogy, say I get drunk and go for drive, miss a stop sign and kill a pedestrian. Even though it was an accident, I'll be charged for at least manslaughter because I created a situation in which such an accident was more likely.

Some police officers habitually create confrontational situations with black men, even in the absence of any evidence of serious wrongdoing. Eric Garner was selling cigarettes illegally (a very minor crime) yet was tackled by officers and put into a dangerous chokehold. The situation was far more likely to result in death or injury than just writing him a ticket at arms length.