This will also come on if you have a bad speaker ..unplug the left rca jack and see if it works....if it dont plug the left back in and unplug the right ,,if that doesnt work unplug both and see if the amp stays on....if this works either you have a bad sub or one of your sub connections is bad or rca jacks are faulty

The protection is on because if any speaker has internal electric problems as open circuit or short circuit, need to be checked