Supposedly it is for 'ice blocks' but I've been told by our station manager that it was instituted years ago after several stowaways were dropped on densely populated areas under the approach path as the gear came down.
We were told it was for ice, (1992), after a large lump of blue coloured ice fell through the roof of an old lady's house. That was a news story at the time complete with pictures of old lady/ice block/hole in roof! I think the theory was that lowering the gear introduced some vibration that might shake the ice off, but the bodies in the wheel well also seems a very likely reason too.