House with many cracked external wall bricks
Cracks are a sign of movement. Movement from 1) a settling foundation, 2) shifting soil, 3) structural wall movement.

1) Obviously, if the foundation is setting the wall will settle. I’d look around the perimeter at the concrete stem walls to see if there are any cracks in the concrete. 

You should look around the perimeter from inside the crawl space too. 

2) I’d check to see if soil is sliding away from the house due to erosion or neighbors installing underground pipes, patio footings, etc. 

3) If the wall is the single wythe brick type (we call them brick veneer walls here in the U.S.) the wood could be shrinking due to drying, dryrot, etc. To check this, simply check to see what is opposite the cracks on the interior walls. 

BTW, we don’t design brick walls with “strong” mortar. The mortar mix is always chosen to be weaker than the brick in compression and tension. That way, when the house moves, (and all houses move,) the cracks will develop in the mortar, which is easier and cheaper to fix than a bunch of broken bricks. 

There is a possibility that the builder was able “to get a good deal” on brick seconds. That is to say, the bricks could have been rejected and sold cheaper. There are tests to help determine the quality of the bricks. Here’s a website to help you get started: 

https://learnwithatul.blogspot.com/2017/09/types-of-field-test-of-bricks.html