May 4, 1970, why did the national guard open fire on and kill four kent state students?

On May 4, 1970, after days of unrest over America's invasion of Cambodia during the Vietnam War, student protesters at Kent State University clashed with Ohio National Guardsmen. The guardsmen opened fire, killing four students and wounding nine others. People were believe the Guard, under the pressure of a rock throwing attack, fired its weapons indiscriminately, killing four people.