Kind of the egg or the chicken situation, annoying atheists were thing at the time already but it quickly became Reddit's "thing", their online popularity (or infamy) came mostly from reddit's own effort in promoting it so hard and corraling the entire demographic in a way that was not replicated anywhere else. There's a reason when even today anyone that think about atheism thinks about reddit specifically, all the astroturf they did backfired on the whole site's reputation spectacularly , it was reddit first big propaganda campaign or at least the first major one before all those people jumped to social justice and orange man bad.