What a horrible start to my 2 week holiday. I went out to feed & water the 3 week old chicks and 7 of the 10 were dead in their run – they were fine yesterday. I have no idea what caused it but 2 of the survivors appear to be on their last legs too.
It doesn’t appear to be the hen attacking them, as they have no marks on them, and no vermin or other birds could have got into the run.
Air an Lot 
That is strange to say the least. You do just wonder what did happened. If the hen did not do it. And there is no evidence of other preys then just who did it? Was it the feed? Or is there something that is in the ground that the chickens just started to peck at? Or, did someone come by late in the evening or in the wee hours in the morning and did something? Okay Donald, my mine is really wondering now.
Also another thought came to my mind. I would put up security cameras around your place. Just a thought.
A dog? We have a collie across the road that goes for hens. Great sheepdog, lovely animal, but has a thing about chickens.
Sorry to see your losses 😦