I just got home to find my glider dead in one of his pouches at the bottom of the cage :( I have no idea what happened.. he had chewed up the bottom half of his own tail pretty badly but no blood, and he had a indent caved in right next to his left eye, a circle about half the size of his eye.

I saw him bouncing around the cage like he usually does this morning before work, he looked to be in perfect health

Their water bowl was full all day, both him and his cage mate were active and friendly and their coats seemed healthy. They are about 3 years old, both neutered at about 1.5 years.

He has never shown any hint of self mutilation in the past

I had them out for play time just the night before.

What could have happened??

Could he have fallen and caved in head in from the fall? It seems unlikely that that would have caused that amount of blunt force trauma and in that round shape.

Could his cage mate have bitten him hard in that spot? I find that so unlikely since there was almost no blood next to the eye, the little round dent didnt look like a bite mark, and because they have never shown any aggression towards each other, especially since both are neutered.

Could there have been some other extremely acute health issue that made him do all that to himself? He has not been sluggish or shown any signs of sickness, and he had a healthy appetite.

I'm in complete shock I just don't understand what could have happened

Are you sure it wasn't the cagemate that chewed up his body after death rather than self-mutilation? If there isn't a lot of blood on the wounds they are probably posthumous. He may have died from some underlying cause, like an undetected health defect, and the cagemate tried to eat the body to hide it. It's not pleasant but it's what many animals do.