https://www.backyardchickens.com/t/587516/my-chicken-has-blown-upAn air leak in her respiratory system. Can happen to humans too (I've seen it in humans), that severe of a leak doubt there is anything you can do, you could try piercing the skin and letting the air out, if it were a more mild leak that would give the body time to heal the leak, but w/ a bad leak there likely isn't enough time. As to why she has the leak, that I don't know. The humans I've seen it in got a tear in the trachea and were on mechanical vents that kept pumping the air in, or got their tracheostomy dislodged and same thing vent kept pushing the air in. Most likely in her she had an unseen issue w/ her lungs (pneumonia, infection, bruise) that formed a bleb that ruptured would be just a guess on my part though (roughly the equivelant of a collapsed lung in a human)
If you do the necropsy, keep the lungs and trachea as intact as possible, my guess is one of those two will have an injury or hole.
Here is a thread about another bird very similar.
I've had a few chicks that had small air bubbles, but never like Mumsy's bird.