The cameraman is standing on the eastern end of the westbound Piccadilly Line platform at Hammersmith, looking east.  On the far side of the bridge, the end of the District Line train can just be seen.  It is standing on the eastbound Piccadilly Line track. There is an operating supervisor in a tango jacket standing to the left of the train on the District Line track.  The train is about to be brought back, wrong way, into Hammersmith station.  This is how District Dave describes the incident for the LUL & Rail forum:

"Wrong Stick" - In LUL parlance 'stick' is a signal. Therefore a 'red stick' is a red signal. 'green stick' is a green signal, 'waiting for the stick' is waiting for the signal to clear and so on.

To be given a 'wrong stick' means that the signal in question has a choice of routes and that the signal has been cleared, but with the wrong route set.  This is a pretty common occurence and although the signal operator should, of course, not set the wrong route, the onus is fairly heavily with the driver to notice the error.  In the case here (see Tom's photo) the signal on the far left would have cleared, but with the Junction Indicator lights lit - showing the route had been set to the Picc.

The driver's actions are then to contact either the controller by radio or phone, or the signal operator via a signal phone and to tell them that he's been given the wrong stick.   He'll be told (usually) not to move and the signal operator will take an emergency release - that is that the signal will go back to red and there will then be a two minute delay before he can reset the route correctly. This delay will be recorded in the Control Room Log as an 'item' against the signal operator.

All clear so far? Good!

However, what happened here is that the driver didn't notice (as occasionally happens) and accepted the wrong stick, so heading (in this case) onto the Piccadilly line and, as has been correctly pointed out toward the tube stock guage tunnel which is, however, some way ahead.  The driver, of course, noticed immediately and had no choice but to stop his train and let the Controller know what had happened.  In this case the item would be against the driver and of course the resulting delay was substantial!

Had he not noticed then, yes, the 'over height detectors' further towards Barons Court would have come into play and prevented the train attempting to go further toward the tunnel.  This was not the case here, however!

So the driver is now on the wrong line with a train load of passengers.  The only way for the matter to be resolved was for the train to make a 'wrong direction move' back into Hammersmith. This involves the manual securing of the points, authorisation being given to the driver once all this has occured and, finally, the train being moved back. All this is a long and involved process, and rightly so as safety must be maintained throughout, hence the substantial delays to both the District and Piccadilly services.

I was working at the time, though on the Edgware Road branch so was not really affected.   Trains coming west were being diverted to Wimbledon, Parsons Green, Olympia rather than adding to the congestion towards Hammersmith (in both directions). The amount of radio traffic was considerable............

The image is very relevant.  What you see is the D Stock train, sitting on the eastbound Picc line (the District line is to its left).  It appears that the train has white lights showing and is therefore preparing to make the 'wrong direction move' I describe above.  Rare shot!   Lucky to be in the right place at the right (or in the driver's case - wrong!) time.

What will happen to the driver?  I speculate that the matter will be the subject of a disciplinary hearing.  At best, the driver can expect to receive a caution which will probably be for a year.  At worst (possibly depending on his past record) he could loose his job.