What causes Rainsford to fall from the yacht in "The Most Dangerous Game"?

Rainsford's fall from the yacht is caused by his lunging for his pipe, which is knocked from his mouth as it strikes a rope after he leaps upon a rail. 
Sanger Rainsford and his friend Whitney travel on a yacht through a moonless night as they are on their way to hunt jaguars "up the Amazon." During an evening conversation, Rainsford and Whitney disagree on their perspectives about hunting these jaguars. Rainsford has no concern whatsoever for the prey that he hunts, while Whitney speculates about the fear that such prey must feel. After this conversation (which acts as foreshadowing), Whitney retires for the night while Rainsford stays on deck since he is not sleepy. "I'm going to smoke another pipe up on the afterdeck," he tells Whitney.
As he relaxes with his "favorite brier," Rainsford is startled by "an abrupt sound." He hears this sound repeated, a sound that he recognizes immediately as the firing of a gun. Then, in his effort to see through the darkness of a moonless night, Rainsford springs quickly onto the ship's rail. As he balances himself, his pipe is knocked from his mouth by a rope. Quickly, Rainsford reaches out for it, causing himself to lose his balance and fall into the "blood-warm waters of the Caribbean Sea." He cries out when he resurfaces from the sea's depths, but no one hears him. Even when he tries to swim closer to the receding yacht, Rainsford knows his efforts are futile. Reaching for the pipe has been a costly error, as he expended a lot of energy doing so but now must swim to shore.