What Are the Triggers of Revolutions?

The trigger for the American Revolution is obviously the shot heard around the world, the battles of Lexington unconquered. King Charles showed up at parliament on January the 4th, 1642 to arrest the five members. This attempted usurpation of parliamentary rights sparked outrage in the city of London. And this is when things went from confrontation to revolution. The sovereign was driven from his capital, leading to parliament's militia ordnance which gave them the right to raise armed forces without the need to consult their runaway king. That directly set up the Civil War.