When the choir first emerged from the jungle to join the larger group, they were wearing their robes. For they to have discarded the robes is a marker of several things. They are forgetting who they used to be, they are forgetting they were part of an organization already, and they are becoming part of the new island society.
The choir had worn their choir-boy cloaks since ending up on the island. Now, they are discarding the cloaks, symbols of the boy's choir and civilization. They are becoming less of a group and more like a tribe. A group implies people who are not necessarily related and can still make their own decisions. A tribe implies a group united by a common chief, who makes most of their decisions for them. Thus, they are beginning their transition from civilized choir boys to savages.