For these designers to be so fashion forward, they all come out w/the same seasonal trends. I heard that the editor of vogue looks at each designer’s collection in advance of fashion shows and decides which look is "in".
Magazine editors and fashion buyers/designers watch the market year round, and watch where the trends are headed. Based on what the magazines pick (and all the buyers/designers also subscribe to each magazine) is what is marketed to the general public. The public (believe it or not) actually pick the trends based on what we buy… and based on what we buy helps determine the trends for the next seasons.
After a trend is determined, notice that the celebrities will start to wear them, they will appear in magazines, and sites like www.thefashionbitch.com, IN Style, etc. will start promoting those trends. When the public sees all the promotion for these items, then they buy it, which helps promote the trend. 
#1 by Fashion Bitch on December 31st, 2009
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Magazine editors and fashion buyers/designers watch the market year round, and watch where the trends are headed. Based on what the magazines pick (and all the buyers/designers also subscribe to each magazine) is what is marketed to the general public. The public (believe it or not) actually pick the trends based on what we buy… and based on what we buy helps determine the trends for the next seasons.
After a trend is determined, notice that the celebrities will start to wear them, they will appear in magazines, and sites like http://www.thefashionbitch.com, IN Style, etc. will start promoting those trends. When the public sees all the promotion for these items, then they buy it, which helps promote the trend.
References :
http://www.thefashionbitch.com
#2 by Chloe K on December 31st, 2009
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Major fashion designers start working on their collection for a specific season as much as year in advance. Why ? It’s not just about the look, but also about the logistics, such as fabric sourcing, runway shows, and production. The runway shows are an important pivotal moment, however. This is where magazine editors and buyers make decisions about the "must-haves" and the "oh, no" concepts and ideas that come together to make up a new trend.
Once the fashion magazines start promoting a particular concept as a trend worth following, some designers, late to the party, may pick up on it. Take the example of "dark lace" for this Fall season. It started with Prada showing dark lace dresses on the runway months ago. Now other designers are starting to include this in their collections. Some designers are interpreting this "trend" in gothic inspired dresses with dark lace. Others are working "dark lace" into elegant evening gowns or even purses.
References :