What is thermal runaway?
All semiconductors, including LEDs, generate heat as a by-product of their operation. The more electrical current the device uses, the greater the amount of heat that needs to be dissipated. Provided there is adequate thermal management, the device will not exceed its maximum operating temperature.
If the provisions to remove heat are insufficient, overheating will occur. When this happens, the ability to conduct electrical increases, generating even more heat. The increased heat output results in an even higher temperature, increasing the ability to conduct current still further. This process, known as thermal runaway, will continue until the device self-destructs.