wall/piston & atom going far away

Dear All,
I am using LAMMPS (14 Feb 2014) and have some questions about wall/piston & shock :
I use fix wall/piston to create shock wave and give the piston a range of velocity:5,10,20,50…During the shocking,I also use fix nve to create a independent condition.
fix 1 all nve
fix 2 all wall/piston zlo pos 1.0 0.0 0.0 vel 0.0 0.0 20.0 units box
The box is ~80A80A300A,containing 112000 atoms.
When running,one or two particles will go far away in the shocking direction.
I change the parameters such as ‘delay’ in ‘neigh_modify’ command via using:
neighbor 2.0 bin

neigh_modify delay 1 check yes
Also,I boost ‘one’ and ‘page’ in ‘neigh_modify’ command
Also,I change the size of box.

However, nothing changes.I wonder how to solve the problem?

Thank you very much for your answer

If you slam a solid with a piston (too hard) atoms
can fly away. What do you see if you visualize
the system? What happens if you slam
the solid more gently? Is your timestep sufficiently