All of the links here on the issue are good.
My personal take on the split is like this:
First off, OpenWRT as a project was very sick before the split. There was development going on with core stuff and bug fixes, but a lot of things were not getting done: Documentation, user-oriented outreach (forum, end-user help, new-dev help, etc), security response was a joke, and other many other issues. Some of the old core devs went AWOL and were really hard to reach and rarely made new commits or worked on the project, but they still had the keys to servers, DNS, and similar stuff.
Then one of the big dev servers over in Europe started going offline, and nobody knew why. Infrastructure like the user-forums, DNS, the bug tracker, and git/svn all would go down for HOURS or DAYS. People tried to reach out to the missing devs and got no reply... for DAYS.
That really kicked things off. People already were not happy with how the current set of project maintainers were running things. There were too few people with the keys to the kingdom and then when stuff broke, nobody was around to fix it.
The old devs are basically holding the name of the project hostage and preventing future progress.
So people forked and that's that.
The new LEDE Project has a lot of problems. I'm not a fan of how they have failed to scope out the project properly, but I think it's a better situation than the old OpenWRT project, which was dying a slow death. At least the new project has a chance at life instead of being smothered by the old devs who refused to share the keys to the kingdom with the active developers.