because you're to young to realize it it dummy and probably too young to be here
There is erectile tissue in the nipples of your tits and the proximity of the edge of the opening in open-front bra stimulates this tissue as you move. Also, the exposed tity-tips are stimulated by the texture of your clothing as well.
As you know, cold can have the same effect.
I once knew a girl whose nipples would elongate nearly 15mm when the tissue was properly stimulated, and would have orgasms if they were gently stroked and sucked.
There's another way. Wear a pair of threadbare white shorts with red panties you can see through the white cloth. Then see how they respond.