They would almost certainly escape into the wild and breed with native or natural species, creating problems like how asian beetles are replacing ladybugs, and earwigs spread like an invasive species, not to mention African honeybees displacing other species of bees.
*hat tip Ace of Spades blog
The problem is that when dark clouds of radioactive ash block out the sun for hundreds of years, cows won't be able to graze but cockroaches will.
They're conditioning you for a nuclear winter or similar, if you're still struggling with the "why" of the past few years.
That actually sounds plausible, sadly