The quick answer is that physics works differently at the smallest scales. Gravity is an incredibly weak field, and other forces come into play when you’re on subatomic scales. That is: “The main idea behind the quantum theory of a Big Bounce is that, as density approaches infinity, the behavior of the quantum foam changes. All the so-called fundamental physical constants, including the speed of light in a vacuum, were not so constant during the Big Crunch, especially in the interval stretching 10−43 seconds before and after the point of inflection. (One unit of Planck time is about 10−43 seconds.)” (from
http://en.wikipedia.org/wiki/Big_Bounce )