Dave Looman
Ansys Employee

A possible explanation is that the surfaces are over-penetrating before contact is established.  Depending on the time step size, it's possible for the surfaces to be not yet in contact at one time point and then have passed through each other at the next time point.  To obtain a good solution at the time of impact it's usually best to model the geometry in contact and then give the appropriate initial velocity to obtain the dynamic effect of impact in conjunction with using very small time steps.