My thought is that in the warehouses, processing plants, etc.. of these places, rats must be plentiful. Use rat poison to kill rats, rats around dog food...... Not too far fetched to think that the food may become contaminated.
Around here, my husband puts out D-Con in the garage. OK for years. Then our cat evidently caught a mouse that had just consumed some D-Con but hadn't had time to die from it. Cat chewed on mouse (which I later found) enough to get very sick and threw up a lot. I think if she had eaten the whole mouse, she would have also be dearly departed.
My guess is that a rats or mice in the pet food warehouse ate rat poison and crawled into the food bin and just stayed there. A LOT of mice or rats . . .
Then, of course, the terrorist trial run theory is also logical. So, plant a Victory Garden and thwart the yahoos in their attempts to kill you. (Of course, we're all so used to pesticides all our lives, they may be barking up the wrong tree.)