Sometimes things happen at the required scale with more spontaneity than planning (as with the first uprisings that started the collection of public shows of discontent in various countries recently) though it is rare. Once there is enough momentum many people join, but the natural reaction is to preserve yourself by not joining in until there is that momentum, which may never happen unless the circumstances are just right at exactly the right time because everyone else is waiting for other people to get started and create said momentum.