Out of control entropy.
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.
In other words, "thermal runaway" describes a process which is accelerated by increased temperature, in turn releasing energy that further increases temperature. In chemistry (and chemical engineering), this risk is associated with strongly exothermic reactions that are accelerated by temperature rise. In electrical engineering, thermal runaway is typically associated with increased current flow and power dissipation, although exothermic chemical reactions can also occur under some conditions. Thermal runaway can occur in civil engineering, notably when the heat released by large amounts of curing concrete is not controlled. In astrophysics, runaway nuclear fusion reactions in stars can lead to nova and several types of supernova explosions, and also occur as a less dramatic event in the normal evolution of solar mass stars, the "helium flash". Wikipedia, Thermal Runaway
Disturbance of Equilibrium
Figure 3.11 Thermal Polarization
Law of Cycles
12.30 - Thermal Radiation and Thermal Vacuum or Cold