Because they are ventriloquists 

Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Dong!
the bell just went off in my head. Beak smack is a sound, as in lip smacking good, right?
I don‘t have any experience with it, but this sounds sort of like clicker training with dogs. If that’s the case, are you not reinforcing negative behavior here? Or are you able to make your beak smack sound like a reprimand?
The head to head displays where they are making the "chu-uh chu-uh" sound is aggression. The "chu-uh chu-uh" vocalization alone indicates aggression although it almost always involves some sort of posturing at same time. If you are doing that back at them, then you are training them to fight you later.I have a video I'm about to link through YouTube since I can't seem to directly download it here. Kotàra (on right) will start to submit by crouching and using a "beak kiss" (?). (Guess I'll use that phrase for now.) When there is no real conflict going on, the behavior is much more subtle and friendly and doesn't seem to include beak clicking. When the sound is used, I have a problem catching it since my phone's mic is not sensitive enough.
If the video or my explanation is confusing, let me know.
The head to head displays where they are making the "chu-uh chu-uh" sound is aggression. The "chu-uh chu-uh" vocalization alone indicates aggression although it almost always involves some sort of posturing at same time. If you are doing that back at them, then you are training them to fight you later.