I saw a post by Joe Dolson pointing to a thread at Cre8asuteforums called “When lots of your visitors go straight to site search“. The thread contains some speculation about what drives people to search – including poor navigation, and a large site. Another driver we see is some people are just search focused – they look for the search box on a new site when they first visit – it is (or should be) easy to find when scanning a page.
The original poster said that he put a second search box at the bottom of his navigation. He found 5% of people used it vs 25% for the one at the top. This is a good idea – it provides easy access to the search for the people that have scanned the page and not found what they were looking for.