What seems to be really important is the expectations of how you interact throughout the protest, and if things combine - such as police mismanagement, territorial boundaries being crossed, no communication between the two sides - then you have expected routines not working out anymore. You see them becoming afraid of the other side. This is when you see tension and fear increasing. This is when violence usually breaks out.

Sociologist Anne Nassauer examines the root causes of violence between protesters and police, and explains why breakdowns in routine, communication and territory - not individual motivations - are key factors in peaceful mass demonstrations breaking down into violent situations.

Anne wrote the book Situational Breakdowns: Understanding Protest Violence and other Surprising Outcomes for Oxford University Press.