When it comes to trends, they definitely don't just pop up out of nowhere. Though it seems like particular styles and items fascinate the general public all at once, it's actually a well-thought out process, starting from various different points. Even for those who think their fashion sense (or lack there of) is completely up to their free will, there are many people influencing every piece that was ever in any store in the world, before a customer even knew it existed. Think of the birth of trends like pyramid.
Downward flow (aka "trickle-down") means that a trend is introduced at the top of the social pyramid, then makes its way down to the markets for the masses to catch on. Celebrities, designers, royalty, and the rich start the trend, then the "regulars" pick up on it later. Trickle-down trends often go unnoticed, but are the most influential, even in this day and age! (See the iconic The Devil Wears Prada quote below)
Horizontal flow (aka "trickle across") means that fashion flows horizontally between groups. The trend is adopted by everyone all at once. These are the trends that seems to take social media by storm nowadays, like Rihanna's Fenty Slides. A non-fashionable example would be Apple's iPhone.
Upward flow (aka "trickle up") starts with the youth or the less rich & makes its way up the age & social pyramids. The perfect example of this? Denim jeans for everyday wear.
To set the record straight about where trends come from, a quote by the fictional (yet iconic) character, Miranda Priestly:
"You think this has nothing to do with you. You go to your closet and you select—I don’t know—that lumpy blue sweater, for instance, because you’re trying to tell the world you take yourself too seriously to care about what you put on your back. But what you don’t know is that sweater is not just blue, it’s not turquoise, it’s not lapis. It’s actually cerulean. And you’re also blithely unaware of the fact that in 2002, Oscar de la Renta did a collection of cerulean gowns. And then I think it was Yves Saint Laurent, wasn’t it, who showed cerulean military jackets? And then cerulean quickly showed up in the collections of eight different designers. And then it filtered down through the department stores and then trickled on down into some tragic Casual Corner, where you no doubt fished it out of some clearance bin. However, that blue represents millions of dollars and countless jobs. And it’s sort of comical how you think that you’ve made a choice that exempts you from the fashion industry, when in fact you’re wearing a sweater that was selected for you by the people in this room. From a pile of stuff.” —The Devil Wears Prada