Usually it is because the control wheel, when pulled back with the left hand, will be turned slightly to the left as the arm is brought back. This is a natural movement, quite unconscious and due to the way the arm is hinged at the elbow. Try it as you sit at your computer; grip an imaginary wheel and pull it back. You will see your hand being forced to go down as the elbow comes back. It takes a conscious effort to train yourself to not do this.
A control stick held in the left hand does the same thing, while if it is held in the right hand should cause the airplane to roll to the right, although a stick is not as bad as a wheel.
Another reason that has some effect on the problem is that the rudder will have been held slightly to the left (in a clockwise turning engine) and as the speed is reduced for the landing, that rudder needs to be taken off.
One way to overcome the problem is to insist on remaining on the centreline, by using rudder initially, following up with aileron if needed.
I usually point out Fred to my students, standing by the runway with a paint pot and brush in his hand. I invite the student to observe the tears in Fred's eyes when he sees them rejecting his lovely centre line.