The answer is unrelated to Murphy’s Law or to bad luck. Note how you hold the toast: at an end, not at the middle. If you hold a toast with both hands horizontally and then release it, the toast will fall butter side up.
A toast held at one end acts like a lever. When you relax the grip, the toast doesn’t fall straight but turns first. Imagine a lever hinged on one end: when released, it falls by turning around the hinges. So the toast turns down first on the hinges of your thumb and forefinger, and only then slips from your hand and falls down.
The moment of rotation pushes the toast beyond a 90-degree angle. A lever dropped while hinging on one end, will gain momentum and pass the vertical-down position. A falling toast doesn’t have enough time to stabilize at the vertical-down position like a pendulum. After initially passing the vertical-down point, the toast appears butter side slightly down and falls thus.
The inertia is not sufficient for the toast to keep turning over. As the toast slips from fingers, the hinges are lost. The toast continues falling in butter-down position.
Additionally, you hold the toast with your fingers removed from the butter layer as much as possible – almost at the bottom. With a thick toast, such a position increases the moment of rotation: imagine lightly holding a parallelepiped at the bottom near the edge.
The rotation effect is pronounced for elongated toasts. Very small round toast, held at the middle, could fall on any side.