Every subtle bounce of the breast while moving, walking, running, etc. Of course, there may be other mechanisms for the damage that bras apparently cause. One such mechanism could be temperature. Breasts are external organs and have a naturally lower temperature. Cancers can be temperature-dependent.
Breast cancer is hormone-dependent.
Temperature can alter hormone function. Breast temperature changes throughout the monthly cycle. All these facts are from the medical literature. Also, just for an interesting experiment, the next time you walk down the street, notice visually how constricting bras are. A physical therapist friend of mine, after reading Dressed to Kill , said that she was amazed at what she saw in her practice at a local medical clinic.