  We investigate the orgin of ``quantum superarrivals'' in the reflection and
transmission probabilities of a Gaussian wave packet for a rectangular
potential barrier while it is perturbed by either reducing or increasing its
height. There exists a finite time interval during which the probability of
reflection is {\it larger} (superarrivals) while the barrier is {\it lowered}
compared to the unperturbed case. Similarly, during a certain interval of time,
the probability of transmission while the barrier is {\it raised} {\it exceeds}
that for free propagation. We compute {\it particle trajectories} using the
Bohmian model of quantum mechanics in order to understand {\it how} this
phenomenon of superarrivals occurs.
