symmetry plane

here is a simple question

I have modeled the reduced section of a  tensile test specimen in 3D (a simple cylinder; falt surface in x-y plane, length in z direction).  I applied a load on one flat surface of the cylinder, and put the opposite flat surface as a symmetry plane. 

When I plot the result after the simulation of a tensile test, the cylinder moved in the direction perpendicular to the application of the load.  I was expecting it only to increase in length in the z direction.