What would cause the supply curve to shift from S2 to S1 as shown in the diagram below?

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.