In the most simple terms -
Vertical -
Track
Understand
this and then think of the net result of the blades not being the same relative to each other.
More prevalent in forward flight due to the above than in the hover.
Lateral -
Mass balance
The lateral or roll axis is the "softest" plane relative to the centre of mass of the aircraft so naturally that is where it becomes more apparent.
More prevalent in the hover as the roll plane is damped in forward flight due to the airflow over the appendages.