Bowing, leaning or bulging in external walls are a result of decreasing wall stability, these issues can be caused by a number of things including:
Prolonged vibrations from heavy traffic or plant machinery
An increase in the floor loads
The original walls being insufficiently thick in comparison to the building height
A lack of restraint between the external walls and the floor joists, beams and partitions
Recent water leak or another moisture problem
Bulging walls occur more regularly in older buildings, semi-detached, or end-of-terrace. These types of properties are more prone to a decrease in wall stability because they will typically have solid walls rather than cavity walls which could have inadequate lateral restraint, a result of the construction methods of the time. You can usually identify a bulging wall visually. Cracks may be evident at the connection of internal walls and floors with the external wall, it is important to know however, that a bowed wall without a crack will still require repairs.