It has to do with finding the contaminant. I believe the additional recalls were made after the discovery of the wheat gluten contaminant.
What baffling if that apparently the trace amounts of the cancer drug/rat poison and the melamine found in the food would not kill or sicken. So they are still trying to figure out what could have sickened the animals.
I have 5 cats and a dog and thankfully hadn't bought any of the recalled food.