So as far as I can deduce, the collapse has been caused by pressure behind the lock wall forcing it into the lock. Very likely to be water pressure. There must have been a void behind the wall, otherwise the water could not have got there. In normal operation, the water remaining in the void when the lock was emptied had only a slight effect on the lock wall, causing a small crack which had been previously filled.
With the flood, (tidal wave?) however, the void filled to a higher than normal level and when the lock was emptied the extra water and leverage at the top of the wall was the straw that broke the camel's back.
The wall of the pound above Napton bottom lock had a slight bulge for some considerable time with no effect until the pound was drained for maintenance.