If dropped straight down, the ball and the board fall at the same rate, so the ball would stay sitting on the tee. But the board moves in an arc, which adds some forward motion to it and causes the tee to tilt an the way down. The ball, using its inertia to try to fall straight down, does not follow the arc. It rolls up the back of the dip in the top of the tee which gives it enough upward force to slow its fall momentarliy until the cup passes under it.
*Looks in rearview mirror at 3:00* Aaahh! Freaky.