All this stuff I have heard from reliable sources or the contestant themselves in an interview. Maybe these reason aren't the sole reasons they were cast on the show, but it really did have a huge effect on them getting on the show.
http://www.ranker.com/list/funniest-reason-someone-was-cast-on-survivor/stokified,
Yau-Man Chan
Casting producer stalked him on ping pong website.
Brian Corridan
He was cast because he corrected Probst's English in casting process.
Gary Stritesky (Fiji)
They needed one person who actually applied to be on that season.
Earl Cole (Fiji)
Amazing race producer randomly asked him to be on that show at a restaurant. They eventually switched him to Survivor.
Todd Herzog (China)
He knew everyone's luxury item that was ever on the show.
Brandon Hantz (South Pacific)
Only cast because he is Russell Hantz's nephew.
Silas Gaither (Africa)
He was cast because he reminded people of Colby Donaldson.
Tina Wesson (Australia)
A lady didn't want to jump out of an airplane. So Tina took her spot on the show.
Russell Hantz (Samoa)
Pirate Master got cancelled, so producers recommended him to Survivor.
Gillian Larson (Gabon)
Producers were annoyed at how many times she applied.