It all depends on how you dropped it, from where you dropped it and how big your slice of toast actually is. All of these factors contribute to the butter side up or butter side down landing position.
In a typical early morning scenario, the toast slips off the side of the table and is given a slight rotation as it starts to fall. As most kitchen tables and work surfaces are about waist height, the toast only manages half a rotation before it hits the floor, butter side down. So, if you want to have your toast landing butter side up, you’d better build your work surfaces twice as high or make smaller slices so that they have time to complete the rotation.