More info needed to even begin to guess at possibilities.
What's your geographic location?
Breed(s) of cows & sires?
Same sire group(s), or calves from different sires affected?
Mineral supplementation program?
Micronutrient analysis (Cu, Se, etc.) on liver tissue from affected calves?
Viral testing performed?
Pasture conditions/forages consumed during the entirety of the pregnancy? You mention drought-stressed hay grazer(whatever that is) after rain - but that's not what these cows were eating for the entire 9 month gestation. Sometimes the 'insult' during fetal development occurrs weeks/months before delivery, and is no longer present or detectible.
You say 'rigid muscles' - but is this actually a muscle problem, or is it a joint/ligament problem, like arthrogryposis?