The Dornier Do 17 recovered from the Goodwin Sands a few years ago was also on its back with the wheels "down".
Given that most of the weight of an aircraft is in the front half, I can imagine that - even after a successful ditching - as the aircraft sinks it effectively "nose dives" and may end up on its back. Over time the buoyancy of the tyres could overcome whatever holds the undercarriage up, and the legs would move towards the surface, giving the impression that the gear was down before the aircraft ditched.