I don’t think this is a right question to ask. It may seem sudden only for those who did not pay attention to it for quite a long time. There always have been women who wanted more than just serving her parents or husband and children. They wanted to prove they are capable of doing more and being equal. Already in the nineteenth century, women demanded more rights for themselves and all that started from separate events and continued in organized protestings. The bright example of this fight is the life story of Anne Hutchinson, who was exiled because of her challenging the way men dominated the church.
The significant changes all over the United States came in a same nineteenth century. It was the time when the lives of all women changed, as well as their social status. At the beginning of the 1820’s a lot of (white) women started working in mills. They were working outside their homes and families, along with men of all social status. Still, the middle-class women were held to the standard and had to stay at home and take care of their families and husband’s needs. So, it did not make a huge impact (as wanted), but it made women more active in fighting for their rights. As they were outside, working with men and performing the same amount of work as men (in dangerous conditions, by the way), they have got the chance to act politically. This started the movement for equal rights, which is continuing even now.
The women's movement in the United States is considered to start officially from Seneca Falls Convention in 1848, although it was of course somehow launched in different communities much earlier.
Another bright example from the history of women’s movement for gender equality is the Women’s Liberation Movement in the 1970s. They were fighting to push the Equal Rights Amendment.
The pittiest thing is actually that after all these years of fighting for equality we still have this disease inside of our society. There still are people who treat women. Differently, they somehow earn less than men (in average) for the same amount of work, and they still have to prove they are equal. And this is after more than 200 years of fighting!