It is totally understandable why dogs knock over trash cans. There are many reasons why a dog may attempt to knock over trash cans. Trash often contains food items or wrappers that smell just like food. That smell is a powerful stimulus for any dog. The trash items a dog pulls out may retain some food odors on them. A hungry or greedy dog that is far from mealtime might knock over trash cans. They simply try to retrieve items out of the trash to eat.