I'm glad you were able to get a necropsy and find out more information.
When I think of Fatty Liver Hemorrhagic Syndrome, I tend to think of overweight hens - too many goodies. However, I am not a vet nor geneticist, so can only speculate - it's very possible that genetics and/or "breed" could play a part. Buff Orpingtons are great, but may not be quite as active as some other breeds so they may not "work off" the extra fat/calories. I have one that is on the move all the time, she is fairly small for a BO. The other one was larger and heavier (about the weight I expected) but she was not nearly as active. She succumbed to internal laying earlier this year, when I opened her up, she did have a bit of a fat pad in the abdomen, but none engorging the liver - she had more fat than a very active Barred Rock that was on the same diet - so I really believe that activity level and diet is the major cause, but genetics (breed) can contribute as well.
I try very hard to not be heavy handed with treats (they are beggars you know). I aim for treats to be no more than 5-10% of their daily intake. Mine get a very small amount of scratch/sunflower hearts (slightly more in winter) then they get something like fresh greens, veggies, fruit and the occasional hard boiled egg. Mine are completely confined to run/coop. If you free range, then that's a bit of a different story - foraging on grass/weeds and bugs are better imho, but you would still want to limit scratch and "extras" like bread, pasta, etc. that some people give.
Again, I'm so sorry about Miss Sadie - she was a lovely lady. BO's are such sweet girls and are a pleasure to have in a flock.