It doesn't always drop into the abdomen. Sometimes, it stays in the oviducts and sort of looks like link sausage in casings. It's a raging infection that cannot be knocked out. The hen cannot live being plugged up, plus it puts a terrible strain on all the organs, sometimes causing the abdomen to fill with fliuid and then, it may progress to the lungs and those begin to fill with fluid. Her heart cannot take the strain on all her systems. They do die from it, every time, that is, unless a vet does a hysterectomy and successfully eradicates all infection from the abdomen.
Infection can start as salpingitis which is bascially an ecoli infection. Some high production birds get "loose" cloacas and feces is sucked back into the oviduct, which of course, starts an infection. Normally, the oviduct closes off when the hen poops, but if that malfunctions, waste matter in the oviduct begins the infection.