Aside from human beings? Believe it or not, it could have been done by wind--if the wind blew some object onto your deck (then off again) that bumped off the plant box! During a gust of wind, my open patio umbrella lifted out of the holder one day, swept several flower pots and two window boxes off the edge of the deck, then flew away--if I hadn't been home, I would not have known that the umbrella had done this!