College football recruits end up transferring for a number of reasons. But lack of playing time is primary among them.
If a player sees that his school has recruited a highly regarded player at his position, he may decide to seek a better opportunity for playing time elsewhere.
The 2014 class has several prospects who could either transfer or force a member of the 2013 class to do the same.
An SEC program has several recruits in its class who are threats to incumbents at their position. A Big Ten school may have a logjam at linebacker, and the emergence of a 2013 signee may cause a 2014 commit to eventually leave the program.