Usually that means there's a short in the speaker wires somewhere. Check the terminals on the back of the unit and the back of your speakers and ensure that there's no strands of metal crossing over from one connector to another. One tiny strand would be enough.

Failing that completely disconnect all speaker wire from the back of the unit. If it still happens then there appears to be an internal problem, and if it doesn't happen you should be able to narrow it down to the wire or one of the speakers.