Sore Spot on Back Causes Hiccups - Why?

People can have all types of stress responses to anything, pain being a common one to illicit stress in the body. The stress response then can be just about anything, but one common one is either mildly hyperventilating through holdings ones breath, or what is known as aerophagia or “swallowing your breath repeatedly” (when you notice pain for example). This may be obvious or so subtle they are hard to pick up on, but this will lead to belching. Other possibilities are disruptions to the vagus nerve (which controls much of the abdominal functions) and/or esophageal spasms - again usually from changes in breathing patterns.

Commonly with self massage as people struggle to apply pressure to themselves they will hold their breath.

There are hundreds of other possibilities, but as this shows up in a good number of psychological conditions (anxiety, panic disorder, depression) once we get the underlying issues under control and in the case of anxiety disorders retrain their breathing to abdominal breathing patterns, the belching is eliminated.