They wear ridiculous clothes just to look silly or unique, listen to music just because it is popular and new, and have horrific attitudes that are incredibly selfish
Real hipsters aren't selfish, it's quite the opposite as most of them are socialists and environmentalists. They are smart, politically aware people embracing the altermondialist movement. To me, this is more important than the artistic aspect of the trend, but that might be a local peculiarity (Montreal area, plenty hipsters around here). The goal is to be live in a more authentic manner, free more neo-liberal shit culture and conservative social norms. I'm a sociology major, therefore I'm supposed to know what I'm talking about (ha, not).
To answer your question, it became cool to be a hipster when ... I don't know, when the world realized intellectuals are much more interesting people than protein shake meatheads. That moment occurred when the hipster-dominated internet culture infiltrated pop culture I believe. That is a personal theory, but the result is the same nevertheless : people copy the general look and attitude, both sold in incredibly overpriced ''indie'' stores (Urban Outfitters, I'm looking at you). Problem is, you can't buy or imitate the deeper philosophy, which is wishing for a better world and not just despising the one we live in. There comes your bunch of idiotic sheepy posers.
Now if some of you were wondering, I have a lot of hipster friends, and while I think the dress code is ridiculous, I agree to their ideas. For the love of god, it's supposed to mock fashion, not to be fashionable.