sagging or drooping of breasts is a natural, inevitable process that happens to all women at some point - except to those with fairly small breasts.
The most notable sagging happens with the process called breast involution, but breasts can start drooping a little at any age, because they do NOT have muscles in them. They have ligaments and connective tissue.
When the gravity pulls the breasts down, those ligaments and the skin can stretch, and so the breast then droops. This depends on the elasticity of your skin and of your ligaments, as determined by your genes and diet, and also on normal aging processes. Obviously large breasts will sag easier since the gravity is pulling them down more. When breasts bounce during active sports, such as tennis, those ligaments can also be stretched or even torn. A good sports bra can minimize that effect.
Breast involution is a process where the milk-making system inside the breast shrinks because it is not needed anymore. This happens either after weaning, or right after pregnancy if the woman does not breastfeed at all, or during menopause. When the tissues inside the breast shrink, and the skin surrounding it doesn't, the breast can look "empty" and saggy. After weaning, a woman's body does usually deposit fat back to the breast (this process takes months), so that breasts will gain their pre-pregnancy size, but sagging usually remains.
Another common cause for sagging is when a woman loses weight. When you lose weight, some of that fat disappears from your breasts. Typically the skin and the ligaments inside the breasts do not retract accordingly, resulting in an 'empty' looking breast that then sags. (You could try prevent this by eating foods that provide extra good nutrition for your skin.)