"Not planned? Whatever reason?" There are huge billion dollar campaigns for what things "are in fashion" this season. For starters, the thousands of ads by fashion brands with the same "current fashion" messaging.
Without that, the majority of the population could not care less what some "small proportion" of idiots consider "experimental/cool/etc". Sub-culture fashion would stay with the subculture (like most of it does, unless it's picked up by the mass fashion industry and promoted to hell).
It's the endless branding, messaging, adverting, PR, articles, etc, that get Joe/Jane Average buying into this season's fasion.
And if some people get enjoyment out of trying out new fashion styles, who am I to judge? No different than me buying the latest video game or checking out the newest programming language or listening to new releases of music.
The people who consciously chace "this season's fashion" are niche [1].
But even more so, people who are bombarded with messages to buy new clothes, and that this style is now obsolete, get this instead, are pretty much everybody though.
[1] Though not that small of a niche: a big percentage of women do it quite a lot (as evident by the success of that side of the fashion industry and peripheral industries like accessories, perfumes, cosmetics, and such), and men have increasingly being doing it ever since the 80s too, especially if we include things like sneakers and so on).
As someone wrote, it's not like 250 million Americans, men and women, each individually by themselves, decided bell-bottoms are a good idea some year in the 70s...
Like, of course we have waves and trends of various things that take hold of popular imagination for some time. Trends start as some niche then people see it taking off and jump on the bandwagon. Sometimes it sweeps the whole country or an entire culture. But it's fun and people get enjoyment from it. And it gives us a common cultural framework to relate to each other with even across vast distances.
Was there no trend you ever participated in that you look back on today and smile about?
It's the same for tech, various gadgets or technologies that are hyped right now have been gathering traction since years but have been picked up by the big brands only very recently. Best example is the trend of using non-x86 Computers, a few desktop/laptop projects have been there including Chrome books. Now Apple picked it up and it's a completely normal thing to buy an ARM laptop.
Looks kind of planned but it's not. It's just that some things fall out of fashion for whatever reasons...