It was the smartphone. Not necessarily the phones themselves, but the wide reaching accessibility. Internet users skyrocketed and if phoneposters we're any indication of the aggregate impact - the quality of conversation deteriorated. And yes, mobile accessible platforms caused a sort of gravity and viola.

Also I'm reasonably sure this is just a truism. "Schismogenesis" in addition to the fact that everybody wants to be on the same page with their community. The nature of the internet in that it's basically just a huge, permanent log of interactions just allows us to observe this shit more easily and remark on it in post.