Militant police and other State forces I would imagine. Death. Something like Kent State.
1. https://www.reddit.com/r/TwoXPreppers/comments/1igugaw/the_5...
This one talks about concerns over the police sending agent provocateurs to protests to instigate violence and let them crack down on the protestors.
2. https://www.reddit.com/r/politics/comments/1j3enrz/trump_ame...
This one is a thread about the government limiting student protests, several comments actually mention Kent State by name
3. https://www.reddit.com/r/AskALiberal/comments/1i1vsp6/why_no...
Here is one asking why there aren't more protests, one answer worries that "protesting in the streets will get people put in prisons being used as slave labor."
These are just the first three results that I found in a moment of searching but I could go on all night. I'm not saying I agree with them or that their thinking is perfect but we can be certain that numerous people are afraid of violent reactions from the state if they try to protest what is happening.