I have no idea what happened the other night at Paddington.
I can offer some insight about the workings of Turbos which may be of help.
On a Turbo the driver can only release all the doors at once, and the driver can only close all the doors at once.This means that the driver couldn't have closed the doors on the rear unit
and kept them open on the front unit.
On a Turbo, the doors can be physically locked shut by hand.This is how it is possible to stop at 6 car Turbo at stops like Appleford. In theory all doors are released, but there is a physical lock preventing them being opened.
So the only way the doors can be locked shut is someone walking through the inside of each carriage with a key at each set of doors (the door out of order light is lit when this has been done). The person locking the doors inside the unit lets them-self in/out the train via a cab door.
Platform staff are trained to physically lock/unlock doors, so it may not have even been the driver's fault. The driver may not even have known as there is no indication in the driving cab that a door is locked.
Driver diagrams tell the driver when to lock/unlock part of a train, so I find it hard to believe that the rear unit was locked out for any particular reason other than some kind of fault.
Now then. Interesting ways to delay a train. I once had a small delay at Hungerford on the way to Paddington. From the cab it looked like I could see two women standing close to each, gesturing, other towards the rear, one inside the train and one on the platform, perhaps wishing the other goodbye. Nothing unusual there. However, I didn't have a clear view of the whole train so I couldn't close the doors. After a few more seconds it became apparent that they weren't waving or hugging each other, but trying to drag a dog on the train. This dog was having none of it, it could not be moved. After a little while longer I thought I would have to get out and help, but just as I stepped off the train the woman on the platform gave the dog a shove and it finally jumped on board. I never did see where they got off. Perhaps they are all still on the train?