I just found one of my girls dead tonight (3 years old). No illness, weight loss, fluffing, feather shed,breathing distress, sneezing, nothing, was doing normal chicken things. I am always looking at droppings and my coops are always clean, nothing out of the ordinary. She went in the nest box earlier today when I was visiting the coop. She was still there tonight but dead. I will be watching the flock with an eagle eye tomorrow.
I did get 6 new bantam hens early last month, but checked them thoroughly. Everyone got wormed and everyone got a round of antibiotics for good measure. No problems with the new girls. Hen (Blanche) was a mixed breed hen and extremely heavy (meaty) so I am hoping, but not happy, that it was a heart attack. But if I have another one go down, it is going to be checked. With the big migration of cow birds here I will be suspecting everything and I am not willing to lose one more of the yard kids.
I hope your situation was a fluke as I am hoping mine was too.