This is speculation without evidence (at least none mentioned) on the part of G&D: "we believe". This is only conjecture based, I believe, on the two other occurrences.
Neither is there any witness testimony regarding this collapse, so we don't really know, which is not to say it could not have happened.
It is my understanding that the part of the stern which broke lose is aft of the rudders.
My personal opinion is that the ... emanations ... from the uncleaned litterbox for Bismarck's cat corroded the rudder shafts.