During intubation the endotracheal tube is accidentally inserted into
the right mainstem bronchus.  As a consequence, the right lung is
over-ventilated and the left lung is not ventilated at all.  A further
consequence is that pulmonary blood flow becomes unequally distributed
between the right and left lungs; a majority of the pulmonary blood
flow goes to the right lung.  why did the pulmonary blood flow change
in this way?

Hi,

The basic question being asked is, why has blood flow been diverted
from the unoxygenated left lung to the well oxygenated right lung?

The vascular system within the lungs adjusts according to the amount
of oxygen present.  In the lungs, the general rule is that if oxygen
tension is low in some alveoli, local blood vessels constrict (hypoxic
constriction) diverting blood to better oxygenated lung tissue.  As
such, when the vessels constrict, the blood flow deminishes - as is
the case in the poorly oxygenated left lung.  It should be noted that
in the periphery (outside of the pulmonary system), the opposite
occurs, and vessels dilate in response to unoxygenated tissue.  This
allows more blood to flow to oxygen poor tissues.

Hope that helps.