I'm very sorry for your loss. Liver disease can have many causes, and often as in many cases a genetic predisposition can be a major factor. Certainly toxins from plants could be one possible cause, but if the liver was visibly cirrhotic then the damage has likely occured over a long period. Many plants that are toxic when fresh are less so when dried anyway. Ragwort is a common hepatotoxic plant. Another source of toxins could be from mouldly hay- aflatoxin from fungi growing on damp hay. Pesticides too.
However, there are also many other things that can cause liver disease aside from toxins- a high calorie diet can certainly contribute, one of my old girls is currently suffering liver failure which personally I think is partly caused by her very poor diet before we adopted her.
But sometimes, organs just become diseased and fail- there are many rare metabolic disorders in most mammals that can stop the liver processing nutrients and medicines and usually harmless environmental toxins correctly.
I'm sorry for your loss, if the vet does suspect poisoning of some kind perhaps they could send urine samples from your surviving piggies to a toxicology lab?
But there is probably no need to be too concerned, hopefully what happened was unrelated to any type of poisoning x
