scorpion stings?
I found this: http://chickenscratchranch.com/tag/scorpion-sting-chicken/
As scorpion stings take affect in chickens, the feet curl and they are unable to hold themselves upright, even while laying down. This lasts for roughly 2-5 days. Most chickens will start showing signs of improvement after 2-3 days and will be back on their feet as if nothing happened after 3-5 days.
Very interesting. This could also explain some of the sudden deaths.