“The dissent is unsigned because Roberts wrote it - it was originally to be his majority opinion. When he switched sides, the conservatives left most of it in place and didn’t sign their names to it to give a clue that it was originally his opinion.”
You nailed it
After further research, I believe the dissent was "unsigned" because it was per curiam explained
here
.............." In legal terms, "unsigned" means the ruling was "per curiam," or "for the court" — simply, that everyone on the panel agreed with the decision and decided to issue a single opinion.
"Generally, an 'unsigned' opinion means the opinion was issued per curiam, meaning no one judge took credit for authoring the opinion, but it represents the opinion of the court," Sarah Ricks, a law professor at Rutgers University who has analyzed types of judicial opinions, wrote us in an e-mail interview.
Ricks added that "it is not unusual for a federal appellate court to issue a per curiam opinion; even the Supreme Court very occasionally issues a per curiam opinion. ... A per curiam opinion does not preclude separate opinions by members of the court but it does represent the reasoning for the judgment of the court." "................