That may be what it is now, but the original cause was spoil d milk. Because Hershey refused to use powdered milk, he had just bought a massive dairy farm and insisted on using fresh milk.

Hershey doesn't use powdered milk. Hershey himself stubbornly insisted on the recipe using fresh milk because he had ignorantly bought up a bunch of cows before getting the recipe nailed down.

And that's why American chocolate is bad, or so the legend goes.

Source: I'm from Pennsylvania and have taken the Hershey factory tour many times since childhood.