I've got lots of questions: What kind of chickens are they? How many have died, and how often is it happening? Does it always happen when you are away? How old were they when you first put them outside? Have you taken one to the vet for a necropsy? Give us as much info as you can think of.

I'd check your feed for signs of moldy or stale feed. If they have been eating the same feed all along, consider changing it. Also, is it possible that a less than friendly neighbor is poisoning your birds when you aren't around? :(

Velvet ~:>

Is there any chance they are getting some chemical that might have been on the ground before it was a pen, or rat/mouse bait or some toxic plant?
I am so sorry for your loss.