Imagine that you are a visitor to a general on a certain board. This general isn't very active because the source material is off air. The general also contains a few shitposting habits and Trip OCs, a motley crew who often engage in antagonistic banter with the anonymous users.
Imagine that one day, you find yourself fixating upon a certain trip. Let's call him Cat. Cat has a reputation for ironic shitposting but also makes on-topic posts. A few people really dislike him, but the chiller members of the general don't dwell on it.
You decide, for a reason known only to you, that you are going to dedicate yourself to making this general an unreadable, toxic pit - and Cat is going to be your vector. One day, exploiting Cat's reputation for shitposting, you start to accuse anonymous posters of being Cat. You do this again. And again. You make sure to do it in ways that are as obviously inaccurate and absurdly illogical as possible. If someone attempts to refute you, you call them Cat as well. If anyone asks you to stop shitposting so they can talk about something on-topic, you call them Cat and do your best to make the conversation go in confusing, anonymous circles until the thread is derailed. Mockingly, you frame your posts as those of a concerned user.
You begin to spin this out into a miniature mythos. Every time someone posts a reaction image, they are Cat because Cat has a huge library of secret reaction images. Every time one of the other trips posts, they are Cat adopting one of infinite sinister aliases in order to talk to himself, because he is a psychopath. Every time someone posts an on-topic opinion or question, they are Cat attempting to waste people's time with some kind of fake discussion.
You exploit naive people in the general who think they can somehow defeat your shitposting by replying to it repeatedly. You spend months doing this and are still at it.
The above story is true.
What kind of human being would do this?
NuMongo is an unspecified later regeneration of the original Mongo. Instead of shitting on Cats, he now sets his sights to making sure that the otherwise-harmless tripLym commits suicide and sometimes shitposts about the show, calling it "d*cter who". It's been speculated that Episode Edition Anon and him are the same person but it's very unlikely.