Salience and specific evidence is a necessary but insufficient condition for these kinds mass outrage conditions. In the end the final trigger is obvious only in retrospect.

As a less politically sensitive topic for most Americans, I point to the Arab spring uprising as a good example. Tensions had been rising for decades, but the final trigger was when Mohamed Bouazizi, a street vendor, lit himself on fire to protest the local police stealing his possessions & threatening his livelihood. A tragic event, for sure, but after decades of brutality and continual tragedy, who could have possibly predicted that that would be the final straw?