box car doors open in transit

why are empty box car doors sometimes open during transit?

Poor pre-depature inspection, only doors restriced from being open are technicaly the plug type doors, but car inspector when walking train should check and secure all doors.
If car leaves the client, with door open, however it can not always be closed by single person, the conductor picking up such cars should know better.

As Dutch said, the only boxcars that are not allowed to move in a train with doors open are plug door cars. And that is because of clearance issues more then anything.

There are no restrictions whatsoever on EMPTY conventional door boxcars being moved with doors open. Even as a conductor, we were never told to refuse to pull a car from an industry with a conventional door open or even ask anyone around to close it. Even when the car department tests an outbound train, they never close conventional doors. They pose no hazard being open. Or to be more precise, they pose no hazard worth the effort to close them. Many doors require the use of a forklift to close or chain type "cum-alongs". Otherwise it would be at least two people to attempt, and then you have to fight with the latch which is not easy to do from ground level if you are short. In any event, that is why you see so many open.