1.An electron moving from the n-type to the p-type material can occur spontaneously because the free electron's energy is released as radiation and it can move to a lower energy state, attached to an atom in the p-type semiconductor. But to move from the p-type to the n-type it has to gain energy from somewhere, and this isn't spontaneous because there is no guarantee of some other process providing this energy.
Think of a ball at the top of a hill: It can move from the top to the bottom while spontaneously releasing energy, but some other process must provide the energy to take it from the bottom to the top. And this analogy provides a basic explanation for why a diode conducts in one direction, but not in the other.
2.When an electric potential is applied these free electrons move in the direction of the electromotive force and we call this electric current