The electric field is directed from n-side to p-side across the p-n junction. When temperature is increased, more covalent bonds break up and hole-electron pairs are generated across the junction. The electric field attracts the electrons towards n-side while it repels the holes towards p-side. Thus the direction of the current due to this movement of holes and electrons is same as the direction of drift current flowing from the n-side to the p-side. Hence the current flowing due to the increase in temperature adds up to the drift current flowing initially in the reverse biased p-n junction diode. Therefore, the total drift current is increased.