Mostly it was really just a handful of youtubers who were highly active and further to the left than we were used to from youtube. Since then they've all become less active (posting longer things less often) and more interested in pursuing niche projects than mass-reach ones. The insistence that this was a movement, instead of a handful of people who mostly knew eachother already, lead to places like /r/breadtube, which has always been a bit lib.