What causes people to all at once buy the same type of thing, such as those sunglasses with the giant gaudy insignias on the sunglasses temples?

How does stuff like this happen? Were these glasses on TV or something?

Or did one person start wearing it and then everyone else did, too?

What goes through the mind when one one sees these in a store and purchases them? Is there a train of thought to it, such as “Oh… those glasses are cool. I love the giant rhinestone sigal mark on the sides”...? or “Oh… there’s the glasses that (insert celebrity name here) wore… I must get them”.

Or is it more like “Oh there’s sunglasses. I’ll buy them” without even noticing the gianty thingy on the temples of it?

For me, it was about a year ago, I saw those things in the store, and thought “Wtf is that?” and then moved on. But then a few months later I notice that more than half the people with sunglasses, have these kind. It started out as an all-woman thing, but now it’s all sexes.

Are these things innately ugly and everyone is pretending they are cool looking, sort of like in the story The Emperor’s New Clothes?

Or are they really cool looking and I’m just scared of gaudy fashion statements?

So how does something like this happen?