The expansion of space (dreamed up to explain the apparent acceleration) but I am not sure I buy into it yet. (religion.blogs.cnn.com)
Until now the favoured theory of catching has been the optical acceleration cancellation model, which assumes that fielders try to position themselves so that the ball is moving directly towards them, and then monitors the ball's apparent acceleration. (newscientist.com)
According to OAC adherents, an outfielder preparing to catch a fly runs along a path that cancels the apparent acceleration of the ball as gravity pulls it toward Earth. (discovermagazine.com)