The opt-out is a speculation that the 2nd set of lawyers can secure a better settlement from the deep-pockets at E&Y and elsewhere.
I would be skeptical of the opt-out. The inertia is clearly with the Trustee, who wants to extend the release to include E&Y. The opt-out lawyers are going to have to get leave to appeal, convince the appeal court, force E&Y to settle with themselves as well. A high bar at every step. Most likely the appeal will simply approve the settlement, and the opt-out folks will be left without a party to sue.