The rain falling behind the shelf cloud causes the air to cool.  As this cold, dense air hits the ground, it is pushed out ahead of the storm.  This cold "dome" of air acts like a barrier to the warmer air ahead of the storm.  As the warmer air gets ingested into the storm's updraft, it hits the dense cold air and is forced upward.  When warm moist air rises, it condenses into clouds.  This results in the shelf cloud seen here.