The most likely cause for a sudden decrease in the amount of milk being purchased in the United States is decreased demand for milk.
When demand for a product decreases, consumers are less willing to purchase it at all price levels, which leads to a lower quantity demanded. The options regarding increased price or increased supply would typically affect the quantity purchased in the opposite direction (either decreasing supply or increasing prices lead to reduced quantity bought). Decreased competition for milk could potentially increase prices or stabilize supply but wouldn’t necessarily lead to a decrease in purchases directly.