After a while even well built dry stone walls for a number of reasons sometimes start to fail and fall down . The ones we are repairing in Renfrew County, as I have said before, have suffered mostly from trees falling on them and from vandals or thieves removing stones.
I suspect there is also another factor that explains places where the walls have mysteriously blown out. Where this wall runs through or near wooded areas, there is a gradual build-up of decayed leaves and plant debris over the years and eventually this creates a fibrous mulch-like mesh to collect inside parts of the wall. This matted material is very porous and I imagine it is similar to having deposits of soil inside the wall. When this stuff gets wet it retains moisture and obviously when it freezes it expands and must push the stones encasing this mesh outward. Obviously the wider the wall the more likely there is to be a build up of material. In the places we have repaired we have tried to remove as much of this material as possible.
It seems ironic that mere plant decay can be the demise of a sturdy stone wall.