Unsteady loads are a major limiting factor for further upscaling of HAWTs considering the high costs associated to strict structural requirements. Alleviation of these unsteady loads on HAWT blades, e.g. using active flow control (AFC), is of high importance. In order to devise effective AFC methods, the unsteady loading sources need to be identified and their relative contribution to the load fluctuations experienced by blades needs to be quantified. The current study investigates the effects of various atmospheric and operational parameters on the fluctuations of α and CL for a large HAWT. The investigated parameters include turbulence, wind shear, yawed inflow, tower shadow, gravity and rotational imbalances. The study uses the DTU's aeroelastic software HAWC2. The study identifies the individual and the aggregate effect of each source on the aforementioned fluctuations in order to distinguish the major contributing factors to unsteady loading. The quantification of contribution of each source on the total fatigue loads reveals >65% of flapwise fatigue loads is a result of turbulence while gravity results in >80% of edgewise fatigue loads. The extensive parametric study shows that the standard deviation of CL is 0.25. The results support to design active load control systems by highlighting the magnitude of CL and α variations experienced by HAWTs, and thus the dCL that needs to be delivered by an AFC system.