The factor that would cause the supply curve to shift from S2 to S1 as shown in the diagram would be an increase of price. What happens is that - given that supply of goods is not constant but shifts from time to time - the price of parts in production increases the cost of production and reduces the profit margin.
As a result, supply falls. It leads to a leftward shift in the supply curve. Similarly, when prices increase the shift is to the right. The factors that affect the supply curve are: input prices, number of sellers, technology, and customer changes in requirements.