For women, the bob cut was the most predominant hairstyle of the Jazz Age. Immortalized in many works but most noticeably in Fitzgerald's "Bernice Bobs Her Hair," the style was a cut that was associated with freedom, women being able to be more sexually seductive, and a more daring view of women than previously articulated. Prominent celebrities such as Coco Chanel and Clara Bow made the bob even more famous. Most of the "flappers," or socialites, of the time period wore the bob cut. Given the mass consumer nature of culture in the 1920s, the bob cut was one of the first examples of a fad or fashion becoming "the rage," and something that "everyone had to have," contributing to its overall fame.
The most usual hairstyle for young women in the 1920s (especially the "flappers") was a very short hairstyle usually called a "bob."
Of course, it is hard to say what made women of that age adopt this hairstyle. It would be hard to explain why women today wear their hair the way they do.
The usual explanation, however, is that short hair was a way of expressing their new freedom. During this time, women started doing many things that they had not previously been allowed to do. In general, they started acting in more independent ways and they started spending more of their lives outside the home. The short hair was a symbol of this new freedom.
As stated in the 2 answers above the main hairstyle of the Jazz age was the bob. However this was only worn by "flappers" for the most part. The majority of "civilized" people considered flappers to be harlets. Watch the original black and white version of "Cheaper By The Dozen" the father gets very upset when the eldest daughter cuts her hair off. Long hair was still considered "appropriate" in the 20's and as the 2 answers above say short hair was a way these women "freed" themselves.
The main hair style for women during the jazz age was most notably known as the "bobbed" style, which was common during 1922. It is a short hairstyle, and more popular amongst younger woman generation. It is more pictured as "curvy waves" and most people thought of it as "flapper's bob".
Why a sudden change in hairsytle? I think the only reason behind it was that the women at that period of time were "freed" from many things as they grow out of dependence to becoming independent themselves and working out in society, so a new change in trend was what the woman trying to express as "freedom", a breakaway from their confined past, and embrace new atmosphere and new levels of fun.