I am not sure what the exact limit is, but Thermal shut offs and limiters are found in many high wattage and high output devices and appliances. It is generally a threshold specified by proprietary engineering, that when exceeded, shuts off the circuit. Essentially, when the wattage is more than the power rating of the speaker, the speaker will enter a shut off or sleep mode. In many live situations where electrical and power amps are unknown, varied and suspect, it may be very easy and common to exceed the threshold of a speaker. Power surges and other electrical spikes and fluctuations can incur this.