Changes in your hormones and even changes in your weight can lead to saggy breasts.
Another cause of breasts sagging is due to not wearing a good sports bra and when you don't wear a good sports bra the breasts move around too freely which then stretch the Cooper's ligaments which lead to the breasts sagging.
You can help firm up your breasts and prevent the breasts from sagging by doing chest exercises.
Simple chest exercises can work the pectoral muscle to produce firmer, less saggy breasts.