What can cause melted chocolate to seize up and become thick?

Melted chocolate can seize up and become thick primarily due to the introduction of moisture. When even a small amount of water comes into contact with melted chocolate, it can cause the sugar in the chocolate to dissolve and the cocoa particles to clump together, leading to a grainy texture and a thick consistency. This process occurs because chocolate is an emulsion, and moisture disrupts that emulsion, causing the components to separate rather than remain smooth and fluid.

In contrast, the other options address different scenarios or conditions that do not directly lead to seizing. For example, overwhipping does not typically apply to melted chocolate and is more relevant to whipping cream or egg whites. Exceeding the expiration date may affect the quality and flavor of chocolate, but it does not cause melting chocolate to seize in the same way moisture does. Heating chocolate gently is actually a preferred method to maintain its smoothness and fluidity, as it helps to control the tempering process. Thus, the introduction of moisture remains the primary factor causing melted chocolate to seize up.