Earlier studies have shown that macro-instability (MI) vortices in vessels stirred by radial flow impellers can be employed to improve mixing performance through guided feed insertion at selected radial positions along the path of a MI vortex, with an associated mixing time reduction of 20-30%. The present investigation provides for the first time an understanding of the physical. mechanisms underlying the MI vortex phenomena, employing a proper orthogonal decomposition analysis to identify and characterize the different flow structures. It is shown that the MI structure is affected by an off-centering perturbation of the core and a stretching mechanism that interact in different ways for low, transitional, and high Reynolds numbers (Re). The different MI vortex frequencies phenomenologically identified in previous works and especially the two simultaneous frequencies encountered for transitional Re are shown to result from a competition between the two mechanisms. The implications of the findings for mixing enhancement at different Re ranges are discussed.