Reasons for disability were numerous. I have read scores of CW disability requests. To the best of my knowledge, they were mostly initiated by two methods. Letters initiated by the soldier himself which would include an endorsement by the regimental or brigade surgeon. Disabilities initiated by a doctor were often written on an official document or certificate. Not all disabilities allowed for a discharge from the army. Although a man was not fit for field service, he was often reassigned to one of the quartermaster shops or factories around the state, preferably near their home. Disability requests had to go through a chain of command beginning with the company commander and progressing through the regimental and brigade commanders up to whomever was authorized to make the final approval such as division or corp commander. The approval process could take a few days or as long as a month.
Do not take it as gospel, because there are thousands of request for disability I have not read, but the two main reasons for disability were due to disease and/or illness or debilitating wounds. Those illnesses which resulted in high fevers would sometime cause blindness, hearing lose, brain damage, etc, etc. Dysentery was another enemy of the soldier. Sometimes prolonged chronic dysentery would leave a man so weak he was not fit for service. Mumps and measles could disable a soldier from active service. Consumption was another illness which eventually required a disability discharge if the soldier didn't die first. There were a plethora of diseases the soldiers were subject to contracting while in service. Another amateur observation is a soldier who spent about 90 days in a hospital and was not recovered enough to return to his unit was often given a disability discharge or at least a furlough to return home.