As hydropower is integrated into the renewable energy system, the turbine components are liable to many loads variation for regulation. The loads experienced under transient operation need to be accounted for and understood to develop adequate mitigation technique and strategies. To identify possible risks occurring during such short and fast transients, we investigate the nonlinear growth and time delay effects of pressure fluctuations, as well as the unsteady flow field evolution for a Francis turbine under load reduction in the part load regime. A two-stage transient process analytical framework is proposed via signal processing and vortex identification methods, including main transient and post-transient stages. In the main transient stage, the dominant frequency of pressure fluctuations within the draft tube shifts from 0.32·fn to 0.24·fn, accompanied by a fivefold increase in the amplitude. Furthermore, low-frequency pressure fluctuations in a wider range are identified (0-2·fn), source of possible resonance of power plant structures. The maximum pressure is reached in the post-transient stage after the end of the guide vane closure and is 50% larger than the maximum value in the main transient stage. When comparing the two components of pressure fluctuations within the draft tube, the synchronous component increases slowly but reaches the peak faster, which can be explained by the evolution of instantaneous vortex structure investigated with proper orthogonal decomposition. The findings are useful to ascertain possible risk factors along with the investigation of the evolution of non-stationary flow field in the context of frequent turbine load variations.