Causes of distortion and movement:
Residual stresses from manufacture of the material, casting, rolling, forging etc. are buried within the material. On machining the balancing stresses may be released and movement or distortion manifest itself. This is particularly true with long thin shafts when stresses from the manufacture of the original bar are progressively released as the surface material is cut away.
Distortion, however small, does occur from heat treatment. This can be greatly reduced by due consideration at pre-manufacturing stages, normalising, stress relieving etc.
Why not machine away the distortion ?
It is important to remember that a bar of steel is not homogenous throughout its diameter . This is reflected in the Limiting Ruling Section for any particular steel and its heat treatment, however even within limited ruling section there is a gradient between surface and core before properties deteriorate below the required and specified level.
If the eccentricity is cut away by machining the symmetry of stresses across the material is destroyed and irreversible distortion will ensue immediately or under service conditions. By straightening the material’s structural symmetry is maintained.
Depending on the component, it might be advisable to include a low-temperature stress relief tempering operation after straightening.