Historically, women in the west have usually worn their hair long. Although actresses and a few "advanced" or fashionable women had worn short hair even before World War I. For example, in 1910 the French actress Polaire is described as having "a shock of short, dark hair", a cut she appears to have adopted in the early 1890s. Her style was not considered very respectable until given impetus by the inconvenience of long hair to girls engaged in war work.
Older people were used to seeing girls wearing long dresses and heavy Edwardian-style hair. Hairdressers, whose training was mainly in arranging and curling long hair, were slow to realize that short styles for women had arrived. Soon barbers in many cities would find lines of women waiting outside their shops, waiting to be shorn of hair that had taken many years to grow.
"A girl should be two things: classy and fabulous!"
As early as 1922 the fashion correspondent of The Times was suggesting that bobbed hair was passé; however by the mid 1920s the Bob style had taken on many various versions, often worn with a side parts, curled or waved. In 1924 the razor cut shingle bob was introduced, it featured hair cut close at the nape of the neck .
Wikipedia and other sources: http://www.1920s-fashion-and-music.com/bob-hair-styles