Simple, politics and control. When you find yourself reacting to the frustration that comes with such things it is better to walk away then allow yourself to succumb.
I remember one time a solid technique did not garner notice from any of the four judges. I asked, they said they didn't see it and it wasn't effective. I bowed politely and when the next command to begin came I applied the same technique with power and got disqualified for excessive force. It worked just fine and this time they observed it.
Now, I don't advocate this type of thing. I was fairly young and inexperienced with karate as I had only been doing it solid for about three years and I was young and a gung-ho Marine, etc. Still no excuse but I decided at that moment to walk away, it was just not worth it.
I believe it was the right path for me. With the type of tournaments gone bad observed on youtube as can be seen at Martial View, i.e. he posted on a youtube video of a contest gone bad, post titled, "Karate Characters."