Sellers don't have control over all of the factors that impact the price of their good. There is something called economic dependence, when the production of a good is dependent on another good or raw material. For example, a cereal brand relies on corn in order to make its product. And if the raw material's price increases, the price of the good naturally increases. If the manufacturer doesn't reflect the price increase, it will lose profits.
Some people think that manufacturers can set whatever price they want. But between competition from other sellers and uncontrollable factors, the price really sets itself.