[XeeThot]

It has nothing to do with the mass of butter....

It has to do with the fact that the toast isn't let go, but it slides out of the plate. It slides outside untill it's center of equilibrium passes over the edge of the plate, and because of that it flips over. Taking in consideration the height that people carry plates and the height of surfaces where the plates are laid upon, there's no time for the toast to complete one (or n) full flip and land butter up.