Here is the story of the attack from three Japanese witnesses:
http://ussgrunion.com/blog/attack-analysis/
From the story, it would seem that the Grunion fired two fish at the Kana Maru. One exploded the other was a dud. It looks like the sub tried to manuver for firing position or possibly lost steerage? the Maru started shooting the 3" gun at the periscope and when "it looked like the sub was surfacing" they hit where they thought the conning tower would come up and "hit" something "a dull water explosion sound" and then a trail of bubbles and a large volume of oil came up.
The blog talks about the bow being missing and a large debris field apparent. Would a loss of depth control after firing the torpedoes and then over compensating for it possibly be the cause?
I'm looking forward to more photographs to try to unravel this mystery.
Chuck