Article is paywalled but I imagine it is something like this:

-Employees said that Musk was a distraction.

-Employees got fired for what they said.

-Employees sued and the NLRB found their complaints had merit, some sort of "hey you're a big boy corporation now so grow up" penalty was assessed.

-SpaceX/Musk is now going after the NLRB.

It's amazing the amount of resources that are wasted soothing the egos of petulant children who happen to be extremely well off.