I think the primary point is that Rosa Parks was not just "somebody" and the event didn't "just happen". There was an effort at the time to bring attention to the segregation of the buses. Black people wanted the situation addressed. They found a way to address it.
From Wikipedia:
Black activists had begun to build a case to challenge state bus segregation laws around the arrest of a 15-year-old girl, Claudette Colvin, a student at Booker T. Washington High School in Montgomery. On March 2, 1955, Colvin was handcuffed, arrested and forcibly removed from a public bus when she refused to give up her seat to a white man. At the time, Colvin was an active member in the NAACP Youth Council, a group to which Rosa Parks served as Advisor. Colvin's legal case formed the core of Browder v. Gale, which ended the Montgomery Bus Boycott when the Supreme Court ruled on it in December, 1956.
Method of segregation on Montgomery buses
Under the system of segregation used on Montgomery buses, white people who boarded the bus took seats in the front rows, filling the bus toward the back. Black people who boarded the bus took seats in the back rows, filling the bus toward the front. Eventually, the two sections would meet, and the bus would be full. If other black people boarded the bus, they were required to stand. If another white person boarded the bus, then everyone in the black row nearest the front had to get up and stand, so that a new row for white people could be created. Often when boarding the buses, black people were required to pay at the front, get off, and reenter the bus through a separate door at the back. On some occasions bus drivers would drive away before black passengers were able to reboard. National City Lines owned the Montgomery Bus Line at the time of the Montgomery Bus Boycott.
Rosa Parks
Rosa Parks (February 4, 1913 - October 24, 2005) was a seamstress by profession; she was also the secretary for the Montgomery chapter of the NAACP. Twelve years before her history-making arrest, Parks was stopped from boarding a city bus by driver James F. Blake, who ordered her to board at the back door and then drove off without her. Parks vowed never again to ride a bus driven by Blake. As a member of the NAACP, Parks was an investigator assigned to cases of sexual assault. In 1945, she was sent to Abbeville, Alabama, to investigate the gang rape of Recy Taylor. The protest that arose around the Taylor case was the first instance of a nationwide civil rights protest, and it laid the groundwork for the Montgomery bus boycott.
In 1955, Parks completed a course in "Race Relations" at the Highlander Folk School in Tennessee where nonviolent civil disobedience had been discussed as a tactic.
Nothing I'm posting is meant as a criticism of Rosa Parks. Segregation was wrong. But the events that made her famous were intentional -- people wanted the situation fixed, and they found a way to fix it.
We may not want to call it "staged", but it was supposed to happen.
It was staged to some degree. I doubt Parks knew that day it would all come together. Doesn’t make it wrong.
Some of the things I’ve read have said that it did happen spontaneously, only with a different woman-an unmarried pregnant young black woman-but the people behind the protest didn’t think that she would be the best “face” of the movement, so recruited Parks, who had long been politcally active, including ties to the CPU.