Fact one: For at least thirty years, police in the US have been deliberately selected for moderate intelligence (you can fail a selection exam just as easily by being too smart as by being too stupid) and for minimum creativity. Police are hired on the basis of their ability to learn complex instruction sets and then follow them without improvisation. This is important: Cops are hired on the basis of their ability to follow orders without question.
Fact Two: Police in the US have, from time out of mind, been taught that their first duty in any situation is to CONTROL THE SITUATION. Force is to be applied in steadily increasing increments until control is achieved. An uncontrolled situation is a dangerous situation. The longer a situation is uncontrolled, the more dangerous it is, and the more force needs to be applied.
So... You take a group of protesters who are determined to refuse to be controlled. That's it; just sit there, regardless of what the cops do. The cops training kicks in; the situation is out of control, and therefore force must be applied until control is established. There are three possible outcomes: The protesters' morale breaks, the protesters' bodies break, or someone on the other side comes to their senses and calls the cops off.
This is not a matter of the cops simply following orders; this is a matter of men who were selected for their ability to follow training doctrines without question acting according to YEARS of conditioning to a situation that, as a matter of doctrine, IS dangerous.
It is a safe bet that most of the administrators who actually have the power to unleash the cops on protesters don't understand this. But they SHOULD. And until they do, this sort of thing is going to keep on happening.