I discovered that if I sleep with the head of my bed too far upright, then the full-face mask that I wear while sleeping was heavy enough to pull my head down on my chest far enough to partially close my throat and restrict my airway. When I fell asleep, my neck muscles would relax and my head would fall forward further than usual because of the weight of the mask. A few minutes later, I would wake up with my chest heaving, trying to pull in a breath. As soon as I got my head upright, the inhalation problem stopped.
Now, I sleep with the head of the bed elevated slightly less, so that my head rolls back into the pillow and not down toward my chest. In this position, gravity pulls the weight of the mask off to the sides. It won't work forever, so I'm looking at other solutions.
It wouldn't surprise me a bit if the esophagus might not be restricted the same way, making it difficult for folks with weakened bulbar muscles to swallow while sleeping, leading to pooling, drooling, etc.