You’re right, it’s not spontaneous, it just happens fast once it takes hold. I remember seeing those ugly chunky glasses on the American Apparel website a good year before I started seeing them in the wild, and when I did see them it was on the hipster fringe of my fellow art students. It wasn’t for like a year after that that they became as prevalent as they did/still are.
So this was a trend that had the groundwork laid by fashion designers, fashion-forward bloggers and other style leaders well before it “spontaneously” erupted all over the scene kids everywhere.
The reason it does happen fast once it takes hold, enough to seem instant, is that as I mentioned above these choices (new glasses etc) take up a lot more time and intellectual energy for people who want to be stylish. In this sense “hipsters” are similar to fashionistas - when you spend a lot of your time, energy and other resources looking for new ways to look good to your own standards, you can adopt new fashions a lot quicker than someone who goes to the mall once or twice a year when they need a new pair of pants. (I’m a lot closer to the latter these days)
But you do have some preferences, an a certain image you desire to signify through your clothing choices though, right? Like, there are probably certain colors and styles you woudn’t be caught dead in, and certain parameters you stay within? That is a style, and you express that style through fashion. It’s not groundbreaking or forward-thinking perhaps, but it’s there.
For you, it’s not a big part of your self image, nor a key area of interest, which is why your choices remain the same for long periods of time. But I’d be very surprised if there aren’t at least some differences between what you wear now and what you wore perhaps 15 or 20 years ago. I’m assuming you’re old enough to have 20 years of adulthood under your belt of course, if that’s not the case sorry.
Basic, middle of the road adult styles do change and evolve, it’s just way, way slower than young, hip, or high fashion styles. For example, when I watch reruns of the early years of Seinfeld, the way Jerry dresses (turtlenecks tucked into belted jeans??) looks somewhat odd, whereas my impression is that at the time it was middle-of-the-road. Maybe I’m wrong and he was being portrayed as somewhat fashion-forward, in which case the datedness makes sense.