I don't think this question could be answered without refering to the Vietnam War and how immensely unpopular this war was in so many quarters of America. Students and many other groups in society were bitterly opposed to a war that had apparently so little to do with American interests and was costing the lives of many American citizens. This of course occurred in the 1960s and protests against this military action began in California in 1965, leading to massive protests and demonstrations in 1968 which featured marches, sit-ins, student strikes and protests.
A key moment in the protest was May 4, 1970, which was when four student protests in Ohio were actualy murdered by Ohio National Guardsnen in the middle of an anti-war rally. Nine other students were shot at this time. This acted to incite greater protest as thousands of students took to the streets protesting about the way that innocent blood had been shed.
Protest in the '60s therefore featured prominently anti-war demonstrations against the involvement of America in a war that seemed to have so little to do with American interests, and yet was causing so many American deaths.