I know fish yawn to stretch there mouths before eating to help loosen up before hunting, I also know they do it to help with gag reflex. I know it sounds weird but they fill up to much on food and if it's starting to come back out they "yawn" to help keep it back. It's like us when we are about to be sick, once it starts going it is hard to stop so they prevent it. I also know some other amphibians like frogs also yawn alot after eating a large meal. But really, who knows...