I don't remember all the details. They never went closed source, there was a difference in opinion between primary devs on the direction the project should take.
Its possible that was related to corporate funding but I don't know that.
Regardless it was a fork where some devs stayed with owncloud and most went with NextCloud. I moved to NextCloud at this time as well.
OwnCloud now seems to have the resources to completely rewrite it from the ground up which seems like a great thing.
If the devs have a disagreement again then the code can just be forked again AFAIK just like any other open source project.