There is no exact science as to how a crack has formed or why without hiring an expert but usually it is due to a slight movement/subsidence of the house.
As we know houses usually settle within the first few months of construction but can also take years. You expect a few minor settling cracks but when they are gaping as shown in the image it could be an indication of a more serious issue.
Filler alone will not fix this type of crack you will need a professional trades person in wall stitching.
PJN Bricklaying & Plastering has over 25 years experience in all aspects of brickwork providing high quality workmanship, good quality service and customer satisfaction.
We can help with all building repairs contact Paul for a free site inspection.