For what it's worth it's the Administrative Procedures Act and it says that even if an agency has the right to make a decision that decision can't be arbitrary and capricious.
Wilbur Ross botched the whole thing but he's being given a do-over.
I think “arbitrary and capricious” would imply contrary to the original intent of Congress, or contrary to simple reason or clear evidence. Otherwise, one person’s “carefully considered” could always be another’s “arbitrary and capricious”, and the law would be pointless, a matter of the court’s whim. The Chief Justice seems to be playing that game.