What can cause a new calf to die?

All sorts of things, from some sort of genetic disorder to a virus or bacteria that the calf picked up in the womb from the cow, or from environmental conditions (from the extremely cold and wet [cold stress] to the extremely hot [heat stress]) that the calf was exposed to when born. Injury from the cow stepping on it or laying on it could also cause some serious damage to the calf. If the cow rejects her calf and he doesn't get colostrum, he could die. Malnutrition of the cow where she doesn't have enough milk or is deficient in a mineral enough that the calf dies when it's born will cause it as well. A prime example of this is selenium deficiency which causes White Muscle Disease.