Do a necropsy on the next purple chicken. Cut open the body carefully and expose the sac that contains the organs. You'l see it will be full of fluid. Ascites is nicknamed waterbelly.
Your broilers are dying from a condition called ascites. I just lost 2 over the last 3 days from this. The tell tale sign is purple combs and skin. Their body cavities are filling with protein rich fluids causing a big increase in blood pressure. Within a day or 2 they will have died from heart...