Not only 1 speaker, but 1 speaker wire can cause every speaker in the car to shut off. What happens is that the wire shorts to ground (or to the other wire) and that causes the internal amp in the deck to go into protect mode. Rarely does the deck ever say anything, it just shuts off all sound. But going by the fact that they only do it at high volumes, I would say that the internal amp on the deck is starting to go. How high do you have the bass/treble? high bass settings with a speaker that can handle more power than the deck can produce can cause the internal amp on the deck to “overwork” and damage itself. Normally it is protected by an automatic shutdown (protect) but it doesnt always work.
Just my .02.
Kenny