Crumbling brickwork on exterior wall
Water is a common destroyer of brick, even if it isn’t freezing and thawing. Water will dissolve bricks and mortar over time. Brickwork must be designed carefully to keep it reasonably dry. The next time it rains, go and look at the damaged areas of brick and see if they are getting splashed from water falling off of a roof edge or something similar. A wider picture providing more context for the location would be helpful for us. 

How is the wall capped, if at all? Is water permitted to enter the wall from above? Brick walls need caps on them, wider than the wall itself, that will shed water. Water entering open joints at the top of a brick wall will ruin it quickly. 

Not all brick within a batch are created equal, especially older bricks that were fired in kilns with uneven heat distribution. Some brick in the batch will be underfired, making them softer and more absorbent, and weaker in wet situations. So a brick can fail while the one sitting right next to it hangs in there just fine. 

I won’t say it’s the only cause of your problem because there are many factors at work and I’m not there to witness them all, but I will say - the condition of those mortar joints is not doing that brickwork any favors at all. In a wet environment brick joints should be finished as below, except for the “raked” and “struck” options.