If I remember correctly, main reason for forking was that Nim BDFL was writing lazy commit messages and old codebase was hard to navigate around. Some small part of community forked the language and trying to rewrite the compiler. According to readme they're not trying to replace Nim, but to create new language with Nim as base. Not sure how usable it's right now or will it ever be.

I also want to point out that Araq quickly took the criticism and his commits look a lot better now.

Also, core Nim team is currently working on their own rewrite of Nim compiler, called Nimony, that should evolve into Nim 3.

Refs:

fork - https://github.com/nim-works/nimskull

commit 'drama' - https://news.ycombinator.com/item?id=32021299#32023998

nimony - https://github.com/nim-lang/nimony