(I like Five Guys' burgers, though I'm not really a fan of the fries.)
It's part of the flow of life up there, where something new opens or gets written up in the Times or New York Magazine or whatever, and then there's this mad hype leading to a huge frenzy and lines out the door.
Most commonly, it was a food fad: frites, Japanese egg/meat pancake-things, creampuffs, Soup Nazi, cupcakes. Usually overpriced, but still relatively accessible. It would run for a while, then burn out or fade away.
What really highlighted the ridiculousness of it all was when NYC got really excited for things that the rest of the country already had and took for granted -- like big-box retail: Home Depot. K-Mart. Bed Bath & Beyond. Whole Foods. Trader Joe's.
You can chalk it up to New York egocentrism, of course (even though it happens everywhere), but it's part of the sense of community up there. Waiting in line (and there's always a line, especially for the food places, which are invariably small), it's a bit of shared suffering, a little bit need to be a part of something fresh or big or cool, with a pinch of "if people are waiting in line, it means there's something worth waiting for."
Even when I lived there, I never bought into the whole "It thing" phenomena, mostly because it was stupid, plus I was oblivious and didn't have very much money. Later, when I saw it, it was as a tourist, which is never the same thing.
Anyway, since I am easily suggestible, I think I will go get a Five Guys burger now.