In "The Most Dangerous Game," what exactly causes Rainsford to fall off the yacht?

Rainsford has stayed on the deck to smoke. As he is standing there thinking of the conversation he and Whitney had just had about the island, he hears a gun shot. He leans over the side of the yacht so he can hear better and judge where the shot came from, and he drops his pipe. When he tries to catch it, he loses his balance and falls into the water.