What causes Phillip from The Cay to fall off the raft and with the sharks?

In chapter 6 of The Cay, Phillip and Timothy are still out at sea drifting along on a raft. Timothy has already warned the young boy that the waters are infested with hungry sharks, just waiting for a man to fall overboard. But as long as they stay on the raft, they should be fine. Phillip, like most children, asks a lot of questions. As Phillip is blind, he wants Timothy to describe what he sees. Timothy happily obliges, telling the young lad about the sky, the vast expanse of sea surrounding them, and the abundant bird and fish life.
Suddenly, Timothy spots an island in the distance. Phillip immediately gets very excited, so excited in fact that he falls off the raft and straight into the shark-infested waters. Timothy dives into the sea at once, quickly rescuing Phillip from a potentially deadly situation.