In most of the cases the reactions are exothermic because all systems want to reduce their net energy and one way to do that is by loosing the $KE$ of the molecules to the surrounding molecules by collisions.
Now suppose that that the molecules of our system cannot emit any radiation so the only way for them to reduce their net energy is by giving their $KE$ to other molecules which are not a part of the system.
Now if I make water with Hydrogen and Oxygen, it results in an exothermic reaction. My question is that hasn’t the $KE$ of the molecules increased when we have mixed Hydrogen and Oxygen together before they pass their $KE’s$ to the molecules of the air? Or in other words does every exothermic system becomes unstable ( because of the increase in the $KE$ of the molecules and hence in net increase in energy ) before it can pass out it’s energy to the surroundings ? And is it that exothermic systems will tend to increase their energy if they cannot pass on their energy to the surroundings ?