Righting yourself and standing up, apparently isn't a natural motion for many adults who haven't learned to swim.

Other than that, someone might have knocked themself out as they fell in, suffered leg injuries, got tangled in something on the bottom, been intoxicated, suffered cold water shock, or fallen into one of the locks or other deeper areas.

I don't know the stats you're looking at, and this is me talking about plausible explanations rather than the actual reasons. But people can drown in puddles if the circumstances are there.