It is not a fall in the price of other goods in general. It is a fall in the price of complementary goods (see our notes, P136).
For example, petrol and cars are complementary goods (if you buy one you will probably buy the other too). If the price of cars falls, then demand for petrol will rise. If demand rises the supply will respond by increasing too: more would be demanded at the same price – hence a rightwards shift in the supply curve.