Why did the Montgomery bus boycott happen?

The Montgomery bus boycott was caused when Rosa Parks, an African American woman, refused to give up her seat on a bus to a white man. Because she refused, police came and arrested her. Soon after, Martin Luther King Jr. led a boycott against the public transportation system because it was unfair. Eventually the issue was brought to the supreme court and racial segregation on buses was deemed unconstotutional. Soon after,King was seen sharing a bus seat with Rev. Glen Smiley, a white man.