That other thing is entropy. Criteria for spontaneity of a reaction. Heat is given off. In this reaction energy is given off. This is an example of an exothermic reaction.
Randomization is also a driving force for spontaneity.
If you examine measure the heat change in this reaction you find that it consumes energy. In other words something other than the heat change is driving the reaction. The other driving force is entropy. Note that there are 2 molecules of reactant by 5 molecules of product. This is an example of an increase in entropy.