When the line voltage goes above 9V again, the left side of the capacitor dives back down to ground, and its other side must do the same, as it cannot change its charge, so it tries to drop through the same amount of voltage. But as it falls the diode clamps it so that if it falls below -0.7V the necessary charge is supplied by the diode through ground. Then the 100k provides the extra path for the current to charge it back from -0.7v to 0v.