I don't understand this at all. Having used the Eurotunnel service many times, all the dividing doors open together and the shutters come up, so why this didn't happen in this case defeats me, unless they had "locked" the middle vehicle.
It's all a bit weird really. As you say, the shutters open automatically so can't have done in this case, I can't believe that there's no CCTV and finally you would have thought they would do last sweep on the train to check it was empty
You can also get phone reception at that point so I'm surprised those trapped didn't try and call Eurotunnel to explain their plight. In fairness, they may of course have done that and were listening to 'Greensleeves' as they held for the 'next available operator' as the recorded announcement directed them to their website :