Why a large Turbine shaft becomes deformed when at rest ?

The force of gravity (weight) applies a constant load to turbine shafts, whether at rest or rotating. This stress always applies downward towards the centre of the earth. When the rotor is stationary, the metal is stressed in one direction only and tends to sag. Initially, the turbine shaft experiences time dependent-anelastic stress.
Ultimately, the sag may become irreversible (permanent deformation), and the resulting creep can lead to extra stress on bearings, vibration, and potential failure of the turbine internals including bearing damages resulting in seizure or catastrophic failure.
 "Creep is the permanent or irreversible, time-dependent deformation of a material under a continuing loads. If the deformation is not managed properly, cracking and permanent failure can result."

How to avoid such potential failure?
It is advised to go for Rolling the shaft which evens out the effect of the gravitational stress on the shaft metal and reduces or eliminates the effects of anelastic stress and sag. If the turbine is left stationary for a long time, the undistributed stress may result in creep, which is irreversible. For this reason, turbines must be rolled regularly as Good operating practice to ensure reliability.