Here's why:

* At the peak of its trajectory, the rocket momentarily stops moving upwards. It's about to start falling back down.

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. At the peak, the rocket's speed is zero, and its direction is changing from upwards to downwards.

It's important to note that even though the velocity is zero at the peak, the rocket still has acceleration due to gravity. This acceleration is what causes the rocket to start falling back down.