We have a number of complicated conspiracy theories regarding the invention and dissemination of the strapless bikini top, but we'll spare you the details. The short version: strapless bikini tops invite nipple slippage. Marisa Miller must've known this when she hit the beach wearing hers, and though the top didn't slip on its own, she was compelled to adjust it, and thus, she flashed nip.
Maybe she felt the fabric move across her breasts, shifting a millimeter or two--just enough to register as a tickle in Marisa's brain--forcing her to address the possibility of slippage. Or maybe she just wanted to check out her nipples. They are beautiful!