It is because your stomach is smaller. Eating too fast or over-eating (and they often go together -- eat too fast and your stomach won't have time to signal the brain that it is full) can lead to pressure/irritation on the vagus nerve. Because the stomach is so much smaller, and the vagus nerve runs along the top of the stomach, it is easier for food to press against it up there at the top then when you had a big stomach and the food had lots of room down at the bottom. The vagus nerve controls all sorts of reactions in the top half of the body, such as coughing, sneezing, runny nose, etc., which is why many sleevers get a runny nose or a cough as a signal that they have eaten too much.
So, your vagus nerve gets super-irritated and it responds with foamies/slimies sometimes. Also, if you eat too fast or too much there is pressure on your esophagus, again, because your stomach is so much smaller and there is nowhere for the food to go. The body may respond to the pressure by "thinking" that there is food or something lodged in the esophagus and that you need to vomit it out, so it will start producing huge amounts of mucus and salivary juices in your mouth and throat in order to prepare for vomiting -- which never comes because you don't actually have something stuck.
This is the way my bariatric nurse explained it to me -- I hope it makes sense and that I have passed it along correctly! Not a medical professional for sure, just trying to relay what was told to me.
