The official reason is that NNN was “brigading”. I have no doubt that anyone merely mentioning another sub was used as “evidence” for brigading.
Meanwhile, there were dozens of subs that were banning all NNN members merely for being NNN members, and this was never considered “brigading”.
This is just more evidence that leftism is a mental disorder.
The political compass is retarded. It's not a real thing. You either believe in authoritarianism or freedom. If you believe in economic authoritarianism and social freedom, you're just a hypocrite.