Yes gliders are colony animals, and MOST do best with others. BUT I have read about breeders have gliders that are alone, happy and refuse a new cagemate.
I think most say single gliders just need lots of attention from a person.
sounds like one most likely attacked. One thing I read, in the wild, gliders
have the option if not happy to leave a colony and try to get a new one.
in captivity they don't have that option.
We had a happy colony of 5 and another of 6. that was two many. we gave 2 of five rehomed and 3 rehomed elsewhere. the 5 never fought or had issues. we heard back several months later one nice female and killed the other nice female.
I wonder about. diet? cage size? quality glider wheel for exercise/activity?
but in the end I don't know that much about why that would happen. Illness of one?
We have an alpha male, he can get mean with the other 5 sometimes. chase, chatter at them, very short spat. but he never hurts them, never any wounds. I have read that a colony can have a structure. One alpha, one pouch protector (crabber) etc. and this organization helps them keep order. so far about 7 years with these 6 and
all is well. but you never know, exotic animals, less know about them.
You could take one to vet for a checkup and ask them, but unless they are very
experienced with gliders they probably wont have an answer for you.