Acceleration and velocity patterns

Hi Experts,

In a typical full frontal crash simulation I plotted a graph between time and acceleration/velocity of one of the parts in my model. The graph was haphazard and not uniform. I mean there were many peaks and valleys in the graph through out the simulation run time ( the results are unfiltered). I would like to know the reason behind this. Could anyone explain why the pattern is haphazard and not uniform ? Is it because of the crash phenomena or because of time step instability or because of interface stiffness......

Thank you in advance..