One of the most common ways things that shouldn't be in food, pet or human, get into the food is cross-contamination. Sometimes, different products are made in the same factory. And they sometimes even used the same equipment. I'm just guessing, but I think a possibility is that there was a new worker who didn't understand which equipment to use for which product. It happens all the time. But that's just my theory.
Regardless of whether the food is cross-contaminated, there are often things in food that you would never want or expect to be there. Fast food is one example, but I'm not going to get into that.
I hope they find out how the poison got in the pet food; I like animals and I don't like to see dead ones.