The below is from oxfordjournals.org
From what it says it sounds like if it really was congenital tremors they wouldn't have died necessarily. Also, most likely only one breed/genetic strain would have been affected. Sorry, just trying to help find the cause.
A new neurological defect in the chicken, congenital quiver (CQ), is characterized by a continuous tremor. At hatching, affected chicks show a rapid tremor, while adult expression varies from a slight quiver to pronounced shaking. Genetic analysis showed the defect to be inherited as an autosomal recessive gene (cq) with complete penetrance. Livability of CQ birds is equivalent to that of their normal sibs. Affected females lay eggs and produce chicks; however, males are essentially sterile