I have often wondered if the phenomenon is partly caused by the same
forces that act on a helicopter rotor (or any other rotating mass) ... a
control input force to the rotor is applied 90 degrees ahead of the
desired output force. Applying that to the marine propeller means that
the right hand prop applying a force (better "bite") at 6 o'clock would
generate a resultant force at 9 o'clock in ahead rotation and 3 o'clock
astern. The force at 3 o'clock would result in the stern moving to port
when backing.

Rick