Perhaps shops that cater to the young fashion conscious can tell you what will be popular next year. See what is carried by H&M, Zara or Top Shop/TopMen in UK this spring.
Also, the tastemakers have sway -- they just previewed the Spring/Summer 2012 clothing? You can find those shows Youtube.
Or you can just blame the economy -- good economy means shorter hemlines for women and shorter men's trunks. Bad economy goes other way.
http://www.guardian.co.uk/lifeandsty...-david-beckham