Why did it take off when it did? What exactly caused it to gain so much momentum during this period?
I know about the lady on the bus who refused to switch seats which resulted an uproar on its own, yet, had it happened 20 years prior, I doubt there would have been as much sympathy for her then.
What changed in society that made the civil rights movement possible?
In fact, there were numerous civil rights movements that would gain momentum and perhaps dwindle out. Just after the Civil War, there was an influx of African Americans taking positions in government that was curtailed by discriminatory legislation. Typically, there is a push for more equitable civil rights just after wars like World War I.
My previous answer about the Red Summer of 1919 that you might enjoy reading some about.
A brief foray into that, the Red Summer of 1919 was a period of civil unrest and protests for Civil Rights. With a bit of conjecture, I'd say this one was severely curtailed by the Spanish Flu pandemic. Which was pretty striking to me this summer.