It was more subtle and interesting than just a fad. It accidentally hijacked a number of our social instincts. In this case, it was disruptive and obvious. It's likely the effect is happening to everyone, just more subtly.

It becomes the mechanism through which a very old human phenomenon — the tendency of a stressed group to adopt a shared physical response to that stress

Basically, our stress responses are subconscious, but learnt. Think swearing when you hurt yourself. It's common, but it's not universal.

In this case, the girls, due to existing stress formed a pseudo social group. The group "leader" had an unusual 'stress' venting method (due to tourettes). The other girls synced to this without conscious involvement. It's the same mechanism that makes us pick up a local accent, or adopt local habits, when we move somewhere new.

In short, fads include this mechanism, but generally have a far more conscious aspect. This was an interesting short circuit, exposing some of the underlying mechanics of the human mind.