These units have a protection device that will trip an internal breaker and shut off to prevent damage. This feature will function normally if: 1. the volume is too loud. 2. You have a stray wire touching either metal on the unit or the wire from an adjacent speaker. Check all of your speaker wire connections. Also, it can't hurt to make sure that the speakers are all properly phased (+ and - connections are consistent on all speakers.)

As an afterthought, I've seen this happen too if the wires are too long or if there have been multiple splices. When the amp cant push the speaker and is overworking itself, the breaker will turn off.