It started when TL;DR made a satirical parody video of the Skeptics.
Which, in turn, led Cognitive Thought, Iron Lizz (who was only doing it to boost her channel, as you'll see demonstrated in TL;DR's response), and someone named Ed to do a response video to it, calling TL;DR an asshole and taking everything in that video way too seriously.
So, TL;DR responds in a rather lengthy video, dunking all of Cog's points and arguments and repeatedly having to explain the jokes that he made in his video.
Of course, Autism is Unstoppable, and Cognitive Thought just wouldn't let up. He kept pushing the point that TL;DR was wrong, despite everyone, even his own fans telling him that he's being fucking stupid.
...which led him to start to insult his own fans and call them TL;DR dickriders.
In all honesty, this is pretty one-sided, as Cognitive Thought was (and still is) in the wrong here. It's just very telling how easily these people get triggered over jokes deriding them, but, as TL;DR demonstrates at the end of his response video, the same people (ie Cog, Lizz, and Ed) would be riding the coattails of a video mocking feminists or the Alt-Right and their livelihoods, since it isn't mocking them and their livelihoods.
So, TL;DR, Cognitive Thought is a fucking sped.