its often an animal who's got a compromised immune system in the first place, but sometimes it'll be a particularly bad form of salmonella, in this case I'm pretty sure it was listed as being a less common form and so the animals in question may have been reasonably healthy but their systems weren't able to cope with that particular form. There are tests that can be performed to confirm the presence of that particular strain in the animal (usuall a fecal sample test), but I don't know if they did it in the case of these cats.