Protesting in a mass movement like many complex behaviors is over-determined and has many "causes." One such cause, seldom appreciated, is to lose oneself in a cause greater than oneself. To renounce your own identity and to identify with the cause (and sometimes with charismatic leadership) is greatly fullfilling. This is especially true if you feel empty, unfocused and lacking in a purpose for living.
Contrary to popular opinion early members of mass movements are middle aged women and adolescent females who have not been oppressed or wronged in any fashion. What they find in the mass movement is a transcendental experience. Further, personally advocating their position (usually not well thought out)reinforces their conviction of the justice and importance of their movement.
I don't suppose anyone reads him any more, but Eric Hoffer wrote a whole book on this called The True Believer. He was writing about the right at the time, but his observations including the finding of middle aged women as important, early members of mass movements is universal and not dependent upon a particular political schema.
It is called idolatry. See my post, #13...