I've had plenty of chickens die suddenly in me, most with no explanation.
Some explanations i have had that may be possible for you also would be...
Poisoning- I live on an old farm and i had 3 or 4 birds die because they started digging a hole where i believe my grandfather may have mixed spray dope for the cherry orchard long ago. This occurred to me after the fact.
Non edible things- chickens eat lots of stuff they should not while seeking grit. Broken glass, nails, plastic, styrofoam. Some of this stuff will kill them
Bad batch of feed- I got a slightly mouldy one once. It made them sick, but I did not loose any.
I would get them separated from the rest of the flock for now, just in case it's catchy, and I always give a couple drops of chicken vitamins right down the throat. I have had some recover.
Hope your girls recover!