Political scientists say that social movements like this happen when you have a few things:
- A sense of "relative deprivation." A group of people has to feel like they have fewer rights (for example) than they think they should and can have. Various things (like WWII, the Cold War, and the Brown decision) made blacks feel like they could get more rights.
- A good leader. This would be, most notably, Martin Luther King, Jr.
- A somewhat supportive environment. In this case, it means that whites in the North (especially) had to be relatively willing to support black demands.
Because these conditions were present, the Civil Rights Movement was able to being at the time that it did.