Any place the core is rotten a hard hit can crack the outer skin (hull): landing off a wave or hitting something in the water, or even while loading on the trailer. If someone put anti fouling paint on the bottom it would apear they left it in the water for long periods of time which would make a better chance for water to be able to get in the core.
This is a stress crack because of rotten core.
What have you done to check your core?
Man I would hate to see you have to pull the floor up later to recore and patch a crack in the hull!!!