Hi
I'm sorry your girl died so suddenly. Fatty Liver Haemorrhagic Syndrome is becoming more common in chickens because we are keeping them more like pets and treating them more. That said there is also a genetic element to it. It is more common in heavier breeds like Orpingtons and Sussex and is usually associated with a diet too high in carbohydrates.... either the owner is feeding them too much scratch or corn or bread etc as well as their regular feed or with mixed grain feeds like Scratch and Peck, the bird is selectively eating the high carb components of the feed and leaving the higher protein pulses. If they have access to wild bird seed, this could well be the problem. Many people feed corn through the winter because they think it helps to keep the bird warm, but the combination of limited exercise due to snow and rain and the high carbs can mean that fatty deposits build up through the winter but the bird may not exhibit problems until they come back into lay in the spring. It builds up over a period of months and because fat is not heavy, the bird may not weigh a lot but still have significant fatty deposits. Cupping your hand between their legs whilst they are roosting is a good way to regularly monitor their body condition as this is where they most often start to store the fat.
I've never heard of high protein being the cause of Fatty Liver and most will advise to feed heavier birds a higher % protein like 18-20% to prevent the disease. Higher protein usually means less carbs since they are the main constituent of any feed.
I hope you are able to figure it out and ensure the health of your remaining flock. A pelleted feed or crumble is best to ensure they get a balanced diet, because it is homogenous and also means that there is less waste as they don't bill it out of the feeder to look for the "best bits".