The most common mechanical equipment adopted in the new generation of pumped-hydro power plants is represented by reversible pump-turbines (RPT), required to rapidly switch between pumping and generating modes in order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources. As a consequence, pump-turbines are required to extend their operation under off-design conditions in unstable operating areas. The paper presents a numerical analysis of the unstable behavior of a pump-turbine operating in turbine mode near the no-load condition. To study in depth the unsteady phenomena which lead to the S-shape of the turbine characteristic, a load rejection scenario at constant and large guide vane opening (GVO) was numerically analyzed by running through the flow-speed characteristic up to the turbine brake region. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating stall (RS) (65.1% of the runner rotation frequency) during the turbine brake operation. These phenomena were characterized by frequency and time–frequency analyses of several numerical signals (static pressure, blade torque, mass flow rate in blade passages). The influence of the development of these unsteady phenomena on the pump-turbine performance in a turbine operation was also analyzed, and the potential causes that generated the S-shaped characteristic curve were also investigated.