Why does my dog poop in his dog bed?

I have a Chihuahua and a Chorkie. They are both trained to use the potty pad in the bathroom when I'm not home. Most days they do very well, but there are times that the Chorkie is pooping in his bed. I keep their beds and blankets clean, but his are washed more often due to this bad habit. The Chihuahua never poops in her bed. They actually switch beds back and forth, so I don't think it has anything to do with smell, but I don't know. That's why I'm asking.