Hello Jess, I'm so so so sorry for your loss. This is the worst part about owning pets- you always outlive them.
my only possible guesses are that she was sick with an internal bug, or perhaps died of loneliness. Depression sets in on gliders when they live alone and despite our best love and affection, they can eventually die.
Other than some sort of cancer (which took one of my gliders) or depression, I can't think of anything...
I'm so sorry <3