I'm not sure we can call this an eating disorder. There are some people who have an issue breathing through their nose and this will affect the way they eat. They are keeping their mouth open so they can get oxygen as they are chewing their food. Then, on the other hand, they were never taught how to eat correctly at the table. They chew their food with their mouth open and make a horrible noise. There are some people who just put way too much food in their mouth and need to keep their mouth open when they chew up their food. Some people feel they are starving and just keep their mouth open and stuff the food down. Others will eat so fast and have so much food in their mouth and try talking while eating. Another bad manner is slurping your food or whatever you eat. There are many bad eating habits and many times people are just in too much of a hurry to sit down and eat correctly, they just don't care.