When people are confined to their beds for long periods, they lose the benefit of gravity in moving their food downward into the stomach. If there is any tendency for acid reflux disease to occur because of other health problems, or if the overall digestive system is sluggish, confinement to bed may be the last straw that causes a person to have acid reflux.
It's also true that people who are confined to bed for long periods usually have serious health problems and may take many medications that contribute to GERD, or they may be elderly and have had undiagnosed GERD for years.