What would cause entropy to decrease in a reaction

The decrease in entropy is caused when the product of a reaction has lesser disorder than that of reactant.  

As we know that the entropy is the measure of system's disorder, the entropy in solid is less than that of liquid and gases. Gases have highest entropy. The thermodynamics, the second law is based on entropy which defines that in spontaneous reaction, there is always increase in the entropy.

If the reactants are undergoing a chemical reaction to produce a product that has lesser disorder, then there is decrease in entropy. For example, if two gases react to form a solid, then there will be decrease in entropy.