Fatty liver isn't common in free ranged chickens. They eat a lot of bugs and grass which are healthy. Fatty liver is linked to High-energy diets in layers that get little to no exercise.Free ranging lowers their risks apparently.I hope you get to the bottom of it soon to prevent any more losses...
After a mysterious disease killed 2 hens I fenced my chickens out of my compost piles and only recently started composting again(my chickens are fenced out of both)