Hot rolling mills suffer of severe torsional vibration in particular working condition (thin thickness). These vibrations are often explained as the result of an exogenous disturbance amplified due to a resonance effect. In this paper, we rather support another interpretation, namely that true origin is to be found in the interaction between the kinematic chain and the plastic deformation of the strip in the roll bite. Thus, the vibrations are the result of an unstable limit cycle due to these interacting dynamics.