Assuming that it is set up correctly: Big assumption
If it shuts off during bass intensive music/movie scenes, it is because the bass requires more power from your avr to reproduce. In addition to that your speakers in the second zone are probably not capable of producing the lower frequency?s that are being sent to it. If your speakers in zone 2 do not have a low pass filter built into them, or if your volume control does not this will cause feedback coming back to the amp and thus shutting it down. Make sure your crossover is set to 80 Hz.
To help narrow down the problem you should disconnect the zone two speakers, set ALL of your speakers to small in the system set up menu, blast a movie with a lot of bass, and see if your system shuts down under these conditions. If it does and it is set up correctly, it is probably just because you have week amps in the denon, which is normal for entry level avr?s.
If you can listen at moderately loud levels, with good bass scenes-think the opening battle scene of Master and Commander, then your avr is fine.
Second, Now just set up the zone two directly to the speakers. If I read you correctly you have two separate areas that have speakers, kitchen and deck, and these have nothing to do with your 5.1 system. Do you have a 5.1 system for ht or jus 4 channels? If you do not have a sub make sure to indicate that in your system set up and MAKE ALL SPEAKERS SMALL!
Now set up each zone one at a time directly to the avr, play music ONLY IN STEREO, and see if it shuts down. Do this to the other zone to see if it shuts down there. If it shuts down in either system, and you set up your avr correctly you have a wiring problem. Now connect one set up speakers to the speaker selector and hook the selector up to the avr, and test one room at a time to see if anything shuts down here. If it shuts down now, you have something wired wrong at the speaker selector box.
99% of the time, you did not set your speakers to small, with the appropriate sub yes/no setting, so your amp being asked to send too large or powerful of a signal than it is capable of. You should add a powered sub if you do not have one.
Good luck