I think your question is, "why did fashion go through a period of above-normal change in the 1920s and 1930s?", but this is based on a faulty assumption that fashion was changing much more slowly before this point. Fashions were changing pretty quickly before World War I, and they continued to change frequently after World War II.

For instance, in 1900 women's fashion featured slight gathering at the front of the bodice, fitted sleeves, low waists, high collars, and trains. Within a few years, a bloused bodice ("blousing" refers to the puffing at the front of the waist) with sleeves that were full near the cuff was fashionable; soon the sleeve fullness migrated up to the shoulder. By the end of that decade, waists were rising, blousing was being streamlined out, and sleeves were fitted again - leading us to "the Titanic period" (as some call it), in which the early 19th century was imitated with narrow skirts and high waistlines, often combined with aspects of avant-garde modern art. Around 1911, it was common for skirts to be plain, but around 1913-1914 a shorter overskirt became fashionable. During the early war years, waistlines could be found dropped somewhat and often loosened, while skirts became fuller - particularly at the hem - and shorter. By the end of World War I, these factors remained, but with a more tubular silhouette that presaged the total lack of waistline that would eventually take place in the early 1920s.

So there is no real way for me to answer this, because there's no speeding-up to explain. If you look into photographs and fashion plates of the two decades prior to your 1918 ones, I think you'll see a pretty steady progression.