Gravity

Although they look far apart, M31 and M33 are locked in a mutual gravitational embrace. Radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.

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.

But... oh. That actually makes sense. Carry on.