n the video in post 43 there seems to be a flash during the landing roll. Could there have been a reverse surge on the right engine and the left engine remained in full reverse? In the early days of the 747, the P&W -3A engines were notorious for reverse surges. On a wet runway, if you didn't cancel the reverse immediately then you were off the side.
Engine surging in reverse is typically caused by high EPR and low runway speed resulting in re-ingestion of the air blown forward by the reverser.
To me just an indicator of the workload in the cockpit at the time rather than a cause