Could it be that someone forgot to remove the bolt on the torpedo? That it was intended to lodge in the ship's wood and then slide off, but the bolt was not removed before the attack?
Not likely. Torpedoes designed to detach were held in place by wooden shear pins -- not steel bolts.
In my "The Spar" forum, we had a member (in Poland), who was doing experiments with cross-drilled sliding-fit pipe pieces, pins of various woods, and a bucket of seawater to soak them in. He reported that softwoods like pine would swell, smear, and jam the release -- but that hardwoods (especially maple) sheared and released cleanly and consistently.
FWIW, in addition to archaeologists and engineers, historian, author (and personal submarine owner/operator), Mark K. Ragan was a "Spar" member. A couple of days after we determined that the deployment angle to reach the actual point of explosion was thirty (30) degrees, Mark posted the now-famous B&W drawings of "Singer's Torpedo -- used for blowing up the Housatonic" -- that he had just found in the Archives! As you can see in my #52, here, that torpedo has a socket at exactly thirty (30) degrees! And it shows a cross-hole for a shear pin... '-)