I have just seen an ITV news clip showing a side view of the event
From this film the jet efflux is impinging normally on the sea as the aircraft decelerates, then suddenly the sea becomes calm underneath (as smoketoomuch said earlier) at which point the aircraft starts to sink BUT also accelerate rapidly forward.
This forward acceleration would suggest to me a running motor but a reduced nozzle angle. Then as the aircraft gets just above the water the sea behind the aircraft is disturbed again by the efflux - apparently pointing fairly well aft,
I would be surprised if that donk stopped,
I would not be surprised if the BOI says the air motor (the power control unit that adjusts the nozzle angle) ran the nozzles aft.
Whatever the cause there was no better place for such goings on.