When they build a cavity wall, they have quite a few steps that they should follow (whether they always do or not is a different story).
A cavity wall should contain a DPC (Damp Proof Course). Which is basically a type of barrier in a wall that keeps moisture out of the inside skin of the wall. Usually this is a plastic type of sheet that they build into the wall.
Of course, we are all human, and sometimes the builder has his head in the clouds and just “forgets” to implement this.
If your builder has built this DPC into your cavity wall, the next step of construction happens, and the rest of the wall is built. In this process, obviously some mortar will drop into the cavity. While they build the wall the cavity has to be cleaned at the end of every day, to avoid these little droppings turning into a little mortar mountain inside your wall. Sometimes this part of the process is dismissed, or forgotten. (We all know how easy it is to just drop and run at the end of the day).
Now you might have a builder or contractor that has implemented the DPC and religiously cleaned it after every day’s work, but he might have been a little aggressive in this, and in the building and cleaning process, the DPC was torn or punctured. As you can imagine, having holes in something made to keep water out is not the best idea.
Of course, one thing that can never be counted out is just plain human error. We have found things like cigarette packets, bottles and even lunch boxes inside of cavity walls. Clearly lunch was not worth trying to dig that out of the wall.
The last thing that can be wrong with your cavity wall is that your builder just didn’t use the right products. Using things like porous plaster under the DPC, or using cheap plastic as the DPC.
In our experience we have come across all of these problems in one wall.