During our travels this summer we often stayed at campgrounds that strongly suggested advance online reservations.
On two occasions we arrived late in the day to find someone else parked in our reserved site. Both times the folks apologized and left.
On one of these, at a county campground outside Casper WY, there were no campground hosts and no internet to get a changed camp site. I could imagine the situation getting out of control with such interactions.
Iowa had a violent episode with several deaths at a state park earlier this year
https://en.wikipedia.org/wiki/Maquoketa_Caves_State_Park_shooting. The police did not identify any motive.
I don't think bears are all we have to worry about.