Why do protests turn violent? It's not just because people are desperate

We have seen ten days of protests in the United States over the death of George Floyd.
While thousands of people have gathered to express their outrage peacefully, some demonstrations have been marred by vandalism and violent clashes with police.
Research has shown that people generally see confrontational protests as unwarranted and ineffectual.
So why do some protests turn violent? And as we watch this mass movement gather pace around the world, what makes people come out into the streets in the first place?