Plywood box under pressure on ABAqus

Hello everyone,

I am working right now on a plywood box simply supported by wedges under a uniform pressure of 1 MPa.

The pressure is applicated through a rigid shell part layed on the top of the box ( a surface to surface contact). The bottom of the box is on contact (surface to surface) with the rigid shell wedges (2 on the corner of the box and 1 on the middle).  

As a boundary condition, all the DOF's of the wedges are constrained.    

Firslty the Plywood box is squeezed under the pressure until she began to bounce back like a spring and separate from the wedges and flying somewhere!!

I really don't understand the rebound behavior of the box. So can anyone help me with this riddle?

Thank you

M.