One guess is that the culprit is squirrels. They commonly go searching for food and dig up planters in spring. Some gardeners have placed hardware cloth over the soil surrounding where the plants are planted. This can be one method of keeping them from digging up the plants. You could also sprinkle cayenne pepper or spray hot pepper spray on the top of the soil, but be careful when you weed the planters that you don't rub your eyes after touching the soil.