This article assesses and estimates the progressive failure mechanism of complex pit-rest secondary toppling of slopes that are located within the vicinity of the Gas Flare Site of Refinery No. 4 in South Pars Special Zone (SPSZ), southwest Iran. The finite element numerical procedure based on the Shear Strength Reduction (SSR) technique has been employed for the stability analysis. In this regard, several step modelling stages that were conducted to evaluate the slope stability status revealed that the main instability was situated on the left-hand side (western) slope in the Flare Site. The toppling was related to the rock column-overburden system in relation to the overburden pressure on the rock columns which led to the progressive instability of the slope. This load transfer from the overburden has most probably led to the separation of the rock column and to its rotation downstream of the slope in the form of a complex pit-rest secondary toppling. According to the numerical modelling, it was determined that the Strength Reduction Factor (SRF) decreased substantially from 5.68 to less than 0.320 upon progressive failure. The estimated shear and normal stresses in the block columns ranged from 1.74 MPa to 8.46 MPa, and from 1.47 MPa to 16.8 MPa, respectively. In addition, the normal and shear displacements in the block columns ranged from 0.00609 m to 0.173 m and from 0.0109 m to 0.793 m, respectively.