Change in the price of a related good:
Goods relate to each other in two ways. Goods are either complements or substitutes.
Complementary goods are goods with joint demand. They are needed jointly before a want could be satisfied, e.g., camera and film. With complementary goods, a steep rise in the price of one will lead not only to a fall in its consumption but a fall in the consumption of the other good too. A fall in the price of one good would lead to an increase in the demand of the other. Substitute goods on the other hand, are goods that only one is needed to satisfy a want/need (not both). For substitutes, a fall in the price of one leads to a decrease in demand for the other and an increase in the price of one leads to an increase in the demand for the other, ceteris paribus.