Another artistic photo, this time of one face of a party wall in a rowhouse. If you look closely (click on the picture to expand it), you’ll see that the plaster is well-adhered to the brick. The plaster didn’t fall off in that one area, it was removed. Why? Because the presence of that long, ugly, and new crack in the plaster suggested that there was damage in the brick wall behind, and the only way to be certain was to remove some of the plaster. If you look closely again you can see the crack quite clearly in the brick wall, although it goes through the mortar joints, not the bricks themselves.
A large part of building investigation is inferring conditions from limited information. Plaster is useful in that it is often attached to wood-joist floors, wood-stud partitions, and brick walls in a fairly rigid manner that forces it to move with the underlying structure. Plaster is both brittle and quite weak, so when it’s forced to move it cracks. The is usually referred to as “telegraphing” in that we don’t see the actual movement, we just get a message. And, just like real telegraph messages back in the day, the news is often bad.