It was what every cultural thing seems to be turning into: internet pranks/memes. Someone saw someone do it, did it themselves, when more people did it, more people did it. Simplest explanation is always the most likely.
I remember hearing a theory that it could have been intended as a viral marketing campaign for the remake of It. Once it started getting bad press, they gave up on the idea, but copycats kept copycatting.
No idea if there's any truth to any of that, but it seems plausible.
