Sounds like Pulmonary Hypertension causing ascites. That bulging rear end is ascites (also known as "water-belly" It's the result of the fast growth rate of CX and the inability of the heart/lungs to keep up with the oxygen demands. If they got chilled in the early days, that will accelerate the development of the condition. It's not a disease and they are safe to eat. I've seen it and I've culled and eaten those kinds of birds.