Here's why:

* Gravity: The only force acting on the ball throughout its flight is gravity. Gravity always pulls downwards, causing a downward acceleration.

* Upward Motion: When the ball is thrown upwards, its initial velocity is upwards, but gravity is constantly working against it. This causes the ball to slow down as it rises.

* At the Top: At the peak of its flight, the ball momentarily stops moving upwards. At this instant, its velocity is zero. However, gravity is still acting on it, pulling it downwards.

* Downward Motion: As the ball begins to fall, its velocity becomes negative (downwards) and continues to increase in magnitude due to the constant downward acceleration caused by gravity.

Therefore, the change in acceleration direction occurs at the moment the ball's velocity changes from upwards to downwards, which happens at the very top of its trajectory.