I always like to relate this to sitting in the bath and holding a bar of soap. Discs are lubricated by fluid which is generated in the vertebrae and are therefore "slippy". Imagine sitting in the bath and holding a wet bar of soap and then squeezing it. It would obviously shoot out of your hand. (I may be exaggerating the analogy a little but you get the picture).
As the person bends forward the pressure between the front discs increases and the pressure at the back decreases just like our bar of soap. It doesn't take much for the disc to be forced backwards causing a "slipped" or prolapsed disc.