The UK has a robust alcohol consumption economy.

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.

The bottom is likely mud and sediment. You sink in with one foot. Then your other foot gets stuck trying to free the first. It's very exhausting to walk through. Sometimes you keep sinking and you're not getting out without help. And the struggle works against you...

There's a serial canal pusher in Bristol.

Canals are not always 3-4 feet deep. Swam in one before and I could only just touch the bottom near the edges, the middle was deeper.

I'm guessing they fall in in the first place because they are drunk. Probably also they hit their head and/or get winded when they fall in.