tobek from my experience, cracks like these are usually caused as a result of vibrations or a shift in the foundation work or superstructure work. It is usually a result of improper foundation being carried out especially when proper blinding and poor mixture of mortar is used.
Also, the quality of the block can cause this issue. When the load on the block is much. it just cracks and this dives down continuously.
When building a house always follow the proper engineering principle and follow your architects and builders recommendation to avoid these issues. Do you know that some houses like this one could have been built from the foundation all the way up without even doing proper DPC/flooring before commencing the superstructure.
What you can do to arrest this situation is to perform some re-enforcements on the crack. You can crack the open the cracked lines, insert a corrugated rod (more like rod that has been welded with branches to it). Insert the rod into the spaces and properly cement it with solid mortar mixture.
I heard that the direction of the line (vertical, slanting or horizontal) can give a pointer on the root cause. Don't know much about that though. I leave it to the Civil engineers to answer. ✌