Hello, I'm sorry for your loss.
It's really hard to tell what might have been wrong (the blood doesnt necessary mean something since gliders sometimes try to hide their mates body by eating them after death in order to prevent predators to be attracted to the colony).
They could have fought, he could have fallen and internally injured himself, he could have been sick (did you notice anything such as constipation, weight loss, change in sleep pattern, change in eating or drinking habits?), it's really impossible to know without a necropsy (and even with one, it's not certain you would have an answer).
I would advise to have your other glider checked by a vet to make sure there is nothing wrong with him, to keep a close eye on him to see any changes, and as a safety measure to clean and disinfect the cage and all toys.