It's because the fabric of your bra is tightly wrapped around the underwire. When it squeaks, it's simply because you're moving in such a way as to cause them to rub together. There's not really anything you can do.

You might try a wider band size. . .which might force you into a smaller cup size as well.

When squeaking happens, it is a sign that your bra is not fitting you well.

You could try to having a fitting at Victoria's Secret or at a higher-end department store.

Your bras squeak because of the wire, I used to work at a Lingerie store and was told the wire was not streached correctly in the bra causing sqeaking, and so if any did squeak we had to exchange them when women brought the back to return. Also i learned that drying your bras by hanging them over your door, or just shower, or wherever you wish is better than putting them in the dryer, sometimes putting them too much in the dryer will cause this too.

Even when i would not put in the dryer they still would squeak so then someone told me to buy a bra that was a little more expensive so i did that and i noticed the chaper bras squeaked when my wire expensive ones dident.