If you need to pass behind a tractor-trailer that is backing up, you should:

Large vehicles have large blind spots to the rear. If you are behind a large vehicle, it is very likely that the driver does not know you are there. Because of this, if you need to pass behind a large vehicle that is backing up, it is always the safest option to wait for the truck's backup maneuver to finish before you pass.
DMV Writen Test