[lammps-users] Sudden change in temperature during shock simulation

Dear Axel and lammps users,
I am trying to simulate the shock compression behaviour in single crystal Nickel at 1.5 kmps velocity. The box dimensions are (-52.860000 -52.860000 -352.40000) to (52.860000 52.860000 352.40000) with periodic boundary conditions. System was equilibrated in NPT ensemble for 10 ps and NVE ensemble for 6 ps respectively. The commands for shock compressions are following:
fix rigidWall all wall/reflect zlo EDGE zhi EDGE units box
fix walls all wall/piston zlo pos -357.4 vel 15 units box

During shock compression, the temperature is increasing randomly after some time. And atoms start to reflect in the opposite direction as shown in the attached image. Color coding is by velocity in Z-direction from 1.5 to -1.5 kmps.
Please explain, what may be the reason for this unusual behaviour?