Quite generally, a system in a metastable well is rendered unstable by thermal and quantum fluctuactions. At high temperature, the former dominate the decay rate, whereas the latter do so at low temperatures. There is a narrow region of temperature where the system shows a crossover between the thermal and the quantum regimes. This crossover resembles a phase transition.
More complicated behaviour is possible as well.
Spin systems provide a highly nontrivial example of the general theory. For these systems, the type of crossover can be tuned by an external field.