There are a number of reasons why a torpedo would float at the end of the run, the most common one being the failure of the self-destruction mechanism.
German torpedoes were equipped with end-of-run self-destruction devices, the purpose of which was to avoid one of them falling into the wrong hands. In case of a failure, or partial failure, the warhead falls off w/out detoinating & this causes the torp to acquire positive buoyancy, hence the floater. But it did not happen very often,
Brian