I mean we have been told. It was described as "a knock" because at the time that's what it looked like. Unfortunately it was a knock that caused underlying damage to the cartilage of the patella - a very rare but extremely serious injury. Also hard to diagnose. and hard to recover from.
There is no "mystery". Just incredible bad luck.
Whilst I’m not a Naval Architect ;
“ a knock “ did for the Titanic.
Bottom line it’s a near-meaningless term that Arthur Rowe might have used back in the day.
Hopefully the international experts ( who use more precise and technical language) will have worked out what went wrong and came up with a practicable plan to fix if .
For his sake and our’s .