There are at least two big factors to viral fashion trends. The first is exposure. The more a piece is worn in the public eye, the more likely it’ll become a big staple for the season. Second, you need a few big names to wear the same piece that season.
The more celebrities that wear the piece, the more exposure you’ll have, and we’re back at rule number 1. However, a viral fashion trend should also be affordable. This means that many brands are making a similar product in a range of prices. If only the celebrities can afford a specific piece, it’s not going to be a viral fashion anytime soon. After all, viral means a lot of people pick it up within a short span of time. It has to be affordable.
Usually it is some celebrity wearing an outfit that may differ from what is currently in fashion, perhaps in some small degree, that catches the eye of those who feast on celebrity matters. If it's a celebrity they would like to resemble, then the clothes item is adopted.
An example is the corset, an undergarment, worn like a belt, almost flaunted, and then this was adopted on the catwalks of fashion houses. It may not be an item of clothing, even. For instance, tattoos are a fashion fad (to be regretted in old age when the image wrinkles and distorts!)