Getting high stress in contact interface

i have simulated a snap fit - By transient nonlinear approach in optistruct, kindly see the attached image. The red colour component engaging with yellow cylinder by rotation at 2.61 rad/s rate .

Rotation about y - axis . only the red colour component was moving and all other components were fixed 

im getting higher stress at interface at the time of engaging (approx.at  0.18 s).  Simulation run for 0.3 sec( for 45 degree rotation )

Can u tell me why this happening ???