Rachel, it is sad, but a real, if painful, part of keeping these birds.
Without a necropsy, it is going to be very difficult to know for certain. As I posted early on, these things never seem to end well. The "gunk" of reproductive infections gets all over inside them and basically, they almost drown in death. It often begins with a puffiness, swelling, strange Frankenstein type movements, followed by lots of sitting alone, hiding alone, tail pumping, sleep eyed look, passing nasty stuff, and ends with an almost comatose state.
Differentiating between ascites, internal laying, egg bound, etc, is not very relevant to me. The bottom line in that the hen is in serious reproductive trouble and without absurdly costly intervention with very little hope for a positive prognosis, the hens often pass away. I personally believe the wasting and passing is quite painful to them as they appear extremely uncomfortable. Putting them down is a merciful act.
We see far too much of this with the utility bred, top laying strains. The reproductive system are simply pushed by their genetics, too far, too fast.