Droopiness of the breast is a common legacy of motherhood, nursing and the force of gravity taking their toll. The effect of pregnancy and the stretch of the breasts with milk causes the fibrous bands which support the breasts in their youthful shape to break down and the skin to stretch. With the subsequent shrinking the unsupported breasts settle into the stretched skin and gravity pulls them down.
Putting on weight and then losing it can have the same effect. So does the aging process, which is why women dislike the appearance of their droopy breasts.