I would suspect heat and/or scorpions.
How are you helping them deal with the heat?
Sorry you have lost some of your girls, Chad. A wading pool is really mandatory for chickens in our desert summer. They need to be able to cool their lizard legs. The mister is an okay idea, but does not do what the wading pool does.
Depending on the roof/cover of your run/house it may be holding in heat.
I lost 2 this year as well - but during that run of 110+ degree days.
It is likely the heat.
Do you have a fan on their nesting boxes (on low)?
Lots of fresh cool water? (I add a frozen water bottle to my founts in the AM)
Also - I let my chickens out on the hottest days - they can then dig in cool dirt...
Sorry about the loss! good luck!
I second what Catherine says about a wading pool or similar structure to hold cool water for the chickens to stand in to cool off. I use the cut off bottoms of the garbage cans I used for compost bins, and put ice in them periodically throughout the day.
Also, sometimes chickens just die without a noticeable cause. Among those I know that keep chickens it happens pretty frequently.
You might medicate to rule cocci overgrowth with the wet hot weather. Definitely do some electrolyte water, and give the girls an afternoon treat of yogurt.