I have given rotational velocity to a shaft through a point and which is kinematically coupled to it's outer surface.This reference point lies in the center of the shaft, which can rotate about its axis only. The results shows that the shaft is rotating properly but the von mises stress values are increasing after every increment. It is not a constant value. Why it is like that?

Abaqus ​​​​​​​Abaqus Explicit ​​​​​​​