Plus each question appears on the main page for yahoo answers I believe. They probably re-index and grab links from that page like hundreds of times a day!
Google spiders high PR websites and websites that add new content regularly more frequently than other sites. Yahoo Answers falls into that category.
All of the websites which are regularly updated are also frequently visited by the Google search engine and this is a proven fact actually and if their sites do manage to have PR, that would create a mega effect on the internet.