The major thrust side on an engine that rotates clockwise is the drivers (left) side of the cylinder, as viewed from the front of the car with the engine in place. The common cause of bore splitting is usually due to detonation, and this "abnormal" combustion is more akin to dynamite being ignited in the cylinder rather than normal combustion pushing the piston down in a controlled fashion. This explosive force sends shock waves in all directions and the thinnest part of the cylinder usually fails. This is why we ultrasonic test cylinder wall thicknesses in all cylinders in 4 locations - to determine if a block has suffered from core shift when manufactured and rejecting ones that do not meet minimal thickness requirements.