Bringing to the discussion to a slightly less learned view.
Bearing in mind that
a: a Cessna 152 cockpit is very small
b: a fair amount of force/body movement is required to open the door
Could it not be that every time this evolution was practiced a knee inadvertently pushed the control column?
I remember when learning to fly (Auster, 1964) being shown that a perfectly trimmed a/c could be controlled by body weight and by putting hands out of the sliding windows. A hand put out of the left widow induced a very gentle turn to the left, and vice versa. Leaning forwards or backwards in the seats induced the appropriate pitch reaction.