In that particular case, four students made an incredibly stupid and dangerous move which resulted in their deaths.
Police were preparing for a possible riot.
Four Students (or at least several in the group of four) moved away from the main group, positioned themselves to the side and began throwing rocks at the poice officers.
Since rocks sound and feel very very similar to bullets (to an officer in gear) - the police DO NOT GIVE THE BENEFIT OF THE DOUBT and quickly responded with fire.
Since then police have worked on ways to better identify the difference between bullets Vs rocks hitting them.
Even still - if you throw rocks at police helmets and shields today, there is still a very good chance they will shoot you till you are dead.
Same goes for spinning around and pulling out a cell phone and pointing it at police.
The reason they will kill you is because many times it IS BULLETS or IS a small handgun.
Its pretty sad those four students put the cops in that position.
They obviously did not think through what they were doing when they chose to hit cops in the head with rocks
Link Update:
http://dept.kent.edu/sociology/lewis/lewihen.htm