From the late 19th century, brick walls were more commonly made with an air gap between two skins of brick, to form a cavity wall. To prevent the skins from separating, metal ties were mortared into the joints.
In modern houses the ties are usually stainless steel but in older buildings these were made from iron; over time this corrodes and disintegrates. As a result the wall may bulge and ultimately collapse.
A good clue is to look for stretcher bond. A special camera can be used to check the state of the ties.
If the ties have corroded, new ties can be inserted by chopping out bricks ant intervals, setting the ties into the inner skin, and then replacing the outer bricks. Alternatively, this can be done by rebuilding the outer skin completely.