They found that the effects of gravity and drag combine to influence the event.
Therefore as the bread falls to the ground it only has the opportunity to achieve half a rotation before reaching the floor, thus landing butter-side down.
NASA astronaut Karen Nyberg, Expedition 36 flight engineer, works to setup the Multi-Purpose Small Payload Rack (MSPR) fluorescence microscope in the Kibo laboratory of the International Space Station. This configuration, along with the leak checks that the crew performed in June, is in preparation for the Aniso Tubule experiment launching on HTV-4 in August. Aniso Tubule will investigate the role of cortical microtubules and microtubule-associated proteins in plant growth while in microgravity.