When you say 'high volume' how high are you at? It's possible that you are driving to the limits of the speaker and it's shutting down. The other one might not be because the level is just SLIGHTLY lower. Because of the nature of audio, the power needed for just a little more volume can be significant even if you can't hear the difference.
Double check that the settings on the speakers are identical. Gain, eq, processing, etc. Make sure you're sending them the same signal, with the same EQ settings. Even though the speakers have built in processing, it's not a horrible idea to run them through a high pass filter to keep low frequencies out of the boxes that you might not need anyway.
The other thing to look at is the power source to the speakers. Is the one that's shutting down on a longer extension cord? Longer extension cords cause voltage drop and that too will trigger a shutdown.
Before sending it in for service, make sure you install the lastest firmware updates on the machines and do a factory reset of the settings. The internal EQ may have been adjusted on one of the speakers which would clearly make it go into protect sooner.
