I have a mystery. I have a house that is about 110 years old. The bathroom wall is old fashion beadboard that runs horizontally throughout the bathroom. One wall has recently been noticed that it has a significant outward bow. I have searched above and below, inside and out as well as under the floor joist. No settling can be found to have suggested the root of this problem. Does anyone have possible causes for this. It is the middle of two boards that are in the middle of the wall. The boards are approximately four feet in length. They are only bowing in the middle. The rest of the wall has not changed.