Jack Goldsmith has some thoughts.

Second, why did the Court grant a case with so many problems, and why did it dismiss?  No way to know, of course, but here is a guess.  UNITE HERE’s cert. petition made it seem like the sky was falling for labor, which the left side of the Court cares about, and then Respondent agreed that certiorari was warranted and made the case seem ultimately to be about employee coercion, which the right side of the Court cares about (but which I do not think is really at stake in the case).  Perhaps this unusual posture, plus a thin though new-ish split in the circuits, plus some poor cert.-pool work by the Justices’ law clerks, led the Court to overlook the obvious procedural and factual problems in the case that would have normally argued against a grant.  And then once the Court confronted the merits, which raised quite knotty problems of statutory construction, these procedural problems loomed larger and offered an easy way out.

H/T Andrew Grossman